\     / 


A 


LABORATORY    TEXT-BOOK 


OF 


EMBRYOLOGY 


BY 


CHARLES  SEDGWICK  MINOT,  LL.  D.  (Yale  and  Toronto),  D.  Sc.  (Oxford) 

JAMES   STILLMAN   PROFESSOR  OP  COMPARATIVE   ANATOMY    IN   THE   HARVARD   MEDICAL  SCHOOL; 
PRESIDENT   OF   THE   BOSTON    SOCIETY   OF    NATURAL   HISTORY 


SECOND  EDITION,  REVISED 
WITH  262  ILLUSTRATIONS,  CHIEFLY  ORIGINAL 


PHILADELPHIA 

P.  BLAKISTON'S  SON  &  CO. 

1012   WALNUT   STREET 
1910 


COPYRIGHT,  1910 
BY  CHARLES  SEDGWICK  MINOT 


BIOLOGY 

LIBRARY 

G 


Printed  by 

The  Maple  Press 

York,  Pa. 


TO 

HENRY  PICKERING  BOWDITCH 

AS  A  TOKEN  OF 

ADMIRATION  AND  LONG  FRIENDSHIP  THIS  VOLUME 

IS  DEDICATED  BY  THE 

AUTHOR 


251988 


PREFACE. 


The  accompanying  volume  is  designed  primarily  for  the  use  of  students  taking 
a  practical  laboratory  course  in  embryology.  It  is  intended  to  direct  the  student's 
attention  to  actual,  original  observations,  to  be  made  by  himself,  and  to  aid  him  in 
drawing  correct  conclusions  from  those  observations.  By  this  plan,  the  student  re- 
peats and  uses  the  actual  methods  by  which  embryblogical  science  has  been  built. 
If  he  pursues  such  a  course  diligently,  he  will  be  able  at  the  end  of  it  to  say  that 
he  knows  of  his  own  knowledge.  To  attain  this  result  is  the  ideal  of  laboratory 
education. 

In  preparing  .the  new  edition,  advantage  has  been  taken  of  the  experience 
gained  with  the  use  of  the  book  by  the  author's  classes,  and  of  valuable  sugges- 
tions from  many  friends.  The  work  has  been  so  extensively  revised  that  it  may  be 
described  as  almost  a  new  book.  Chapters  III  to  VI  have  been  entirely  recast  and 
rearranged  so  as  to  conform  to  the  chronological  order  of  development,  an  arrange- 
ment which  it  is  believed  most  readers  will  prefer  to  that  adopted  in  the  original 
edition.  Chapter  II,  on  the  early  development  of  mammals,  has  been  considerably 
expanded;  not  with  the  object  of  giving  a  comprehensive  treatment  of  the  subject, 
but  rather  with  the  intention  of  aiding  the  student  to  get,  in  connection  with  his 
laboratory  work,  a  connected  story  in  his  mind  of  the  development  of  the  principal 
organs  and  systems  of  the  body.  Some  new  sections  will  be  found  also  in  Chap- 
ter I.  A  considerable  number  of  the  figures  are  replaced  by'new  ones.  The  total 
number  of  illustrations  has  increased  from  218  to  262.  With  these  changes  it  is 
hoped  that  the  second  edition  will  deserve  a  continuance  of  the  favor  shown  to 
the  original  issue. 

The  author  takes  much  pleasure  in  acknowledging  gratefully  the  invaluable  as- 
sistance afforded  him  by  members  of  his  laboratory  staff,  and  wishes  to  call  at- 
tention especially  to  the  very  admirable  original  illustrations  which  have  been  fur- 
nished especially  for  the  book  by  Drs.  J.  L.  Bremer,  F.  P.  Johnson,  F.  T.  Lewis, 
R.  E.  Scammon,  and  F.  W.  Thyng.  Special  mention  must  be  made  also  of  the 
figures  from  models  made  by  Messrs.  W.  W.  Behlow,  G.  C.  Coe,  L.  M.  Fergu- 
son, C.  A.  Hedblom,  and  A.  R.  Kilgore,  students  in  the"  embryological  course  at 
the  Harvard  Medical  School. 

A  large  majority  of  the  illustrations  are  from  the  Harvard  Embryological  Col- 
lection, without  which  this  work  would  not  have  been  possible.  The  author  requests 
those  who  use  this  book  to  communicate  to  him  any  suggestions,  which  their 
experience  may  lead  to,  for  improving  it. 

CHARLES  S.  MINOT. 

HARVARD  MEDICAL  SCHOOL,  May  28,  1910. 

vii 


CONTENTS. 


CHAPTER  I. — GENERAL  CONCEPTIONS, i 

The  Segmented  Animals.  Metamerism,  2 

The  Vertebrate  Type  of  Structure,  2 

The  Principal  Modifications  of  the  Vertebrate  Type,  7 

Definition  of  Anlage,  9 

A  Summary  of  Embryological  Development,  10 

Cytomorphosis, 1 1 

Comparison  of  Larval  and  Embryonic  Types  of  Development,  16 

Germ-layers,  * 18 

The  Relations  of  Surface  to  Mass, 20 

Classification  of  Glands, 23 

The  Law  of  Unequal  Growth,  24 

Germ-cells, 25 

Sex, 27 

The  Theory  of  Heredity,  28 

The  Law  of  Recapitulation, 29 

Arrest  of  Development, 31 

CHAPTER  II. — THE  EARLY  DEVELOPMENT  OF  MAMMALS, 33 

The  Spermatozoon, 33 

The  Fully  Grown  Ovum  Before  Maturation, 34 

Ovulation,  35 

The  Maturation  of  the  Ovum,  .  .  . 36 

Impregnation  of  the  Ovum, 38 

Segmentation  of  the  Ovum,  42 

The  Blastodermic  Vesicle,  45 

The  Embryonic  Shield,  47 

Growth  of  the  Embryo  and  Separation  of  the  Yolk,  49 

Origin  of  the  Mesoderm, 51 

The  Primitive  Axis, ' 52 

The  Notochordal  Canal,  ; 53 

The  Notochord, * 55 

The  Ultimate  Fate  of  the  Notochord, 56 

The  Archenteron, 57 

The  Oral  and  Anal  Plates,  58 

The  Digestive  Canal,  59 

The  Yolk-sac,  63 

The  Origin  of  the  Nervous  System, 67 

The  Structure  of  the  Medullary  Canal,  ' 69 

Origin  of  Nerves, 72 

ix 


x  CONTENTS. 

PAGE 

The  Spinal  Cord  and  Brain,    

Plakodes,    

The  Nasal  Pits  and  Olfactory  Nerves,  . 

The  Eye,   

The  Otocyst, 

The  Early  History  of  the  Mesoderm,  . 
Somatopleure  and  Splanchnopleure,  .  . 

The  Embryonic  Ccelom, 

The  Mesenchyma,   

The  .Origin  of  the  Blood-vessels  and  Blood,  9° 

The  Blood-corpuscles,    • 

The  Origin  of  the  Heart,    96 

The  Germinal  Area,   9° 

The  Main  Vessels  of  the  Area  Vasculosa,  ...  97 

The  Aortic  System,   99 

The  Venous  System,   102 

The  Lymphatic  System,     I05 

The  Liver, I07 

The  Pancreas, I07 

The  Excretory  Organs, Io8 

The  Urogenital  Ducts,    .no 

The  Allantois, IIX 

The  Trophoderm,    "4 

The  Umbilical  Cord, ......,;..,..  115 

The  Chorion  and  Amnion,    1 1 7 

CHAPTER  III. — THE  HUMAN  EMBRYO, "8 

Calculation  of  the  Age  of  the  Human  Embryo,      .  .  . .  118 

The  Classification  of  the  Early  Stages,    119 

Hypothetical  Development  of  the  Blastodermic  Vesicle  in  Primates, 122 

Relations  of  the  Embryo  to  the  Uterus:  the  Two  Stages, 124 

Ovum  of  a  Monkey  in  the  Second  Stage, 127 

Human  Embryo  in  the  Second  Stage,    128 

The  Embryo  of  a  Gibbon  in  the  Third  Stage,   131 

Human  Embryo  in  th  Fourth  Stage  with  the  Medullary  Plate, 134 

Human  Embryo  in  the  Fifth  Stage  with  Open  Medullary  Groove, 136 

Human  Embryo  in  the  Sixth  Stage  with  Medullary  Canal,   . .  . 137 

Human  Embryo  in  the  Seventh  Stage  with  One  Gill-cleft  Showing  Externally, ...  1 40 

Human  Embryo  in  the  Eighth  Stage  with  Two  Gill-clefts  Showing  Externally, 141 

Human  Embryo  in  the  Ninth  Stage  with  Three  Gill-clefts  Showing  Externally,  .  .  .- 143 

Human  Embryo  in  the  Tenth  Stage  with  Four  Gill-clefts  Showing  Externally, 146 

Human  Embryo  in  the  Eleventh  Stage  with  the  Cervical  Sinus  in  Formation, 147 

Human  Embryos  of  the  Fourth  Week  to  the  Fourth  Month,    148 

CHAPTER  IV. — STUDY  OF  THE  SEGMENTATION  OF  THE  OVUM  AND  OF  THE  BLASTODERMIC  YKSI- 

CLE  IN  MAMMALS,    ; 1 60 

The  Maturation,  Fertilization,  and  Segmentation  of  the  Ovum  in  White  Mice,    160 

Method  of  Obtaining  Blastodermic  Vesicles  from  the  Rabbit,   166 

Study  of  Rabbit  Blastodermic  Vesicles  in  Alcohol,    167 

CHAPTER  V.— STUDY  OF  YOUNG  CHICK  EMBRYOS, 1 74 

Method  of  Obtaining  Embryos,   ; 1 74 


CONTENTS.  xi 

PAGE 

Embryo  Chick  with  Eight  Segments  (About  Twenty-eight  hours'  Incubation), 176 

Embryo    Chick   with   about  Twenty-four  Segments  and  Three   Gill-clefts  (About  Forty- 
six  Hours'  Incubation), 197 

Embryo  Chick  with  Twenty-eight  Segments, 199 

The  Study  of  Transverse  Sections, 199 

Horizontal  Section,      214 

Histological  Differentiation  of  the  Chick  Embryo  with  Three  Gill-clefts, 216 

CHAPTER  VI. — STUDY  OF  PIG  EMBRYOS,    219 

Methods  of  Obtaining  Embryos,    219 

The  Making  of  Serial  Sections,   220 

Selection  of  the  Planes  of  Section  and  the  Stages  for  Practical  Study,    220 

The  Study  of  the  External  Form,   221 

Pig  Embryo  of  7 . 5  mm., 221 

Pig  Embryo  of  10  mm., ". 223 

Pig  Embryo  of  15  mm., 225 

Pig  Embryo  of  20  mm., 226 

Pig  Embryo  of  7.8  mm.     General  Anatomy,  228 

Pig  Embryo  of  12  mm.     General  Anatomy,  231 

Pig  Embryo  of  6  mm.     Studied  in  Sections,    246 

Pig  Embryo  of  9  mm.     Studied  in  Sections,    250 

Pig  Embryo  of  12  mm.     Studied  in  Sections', 259 

Study  of  Transverse  Sections, 261 

Study  of  Sagittal  Sections, 290 

Study  of  Frontal  Sections, 296 

Pig  Embryo  of  17  mm.     Study  of  Sections, 303 

Frontal  Section  of  the  Umbilical  Cord,    310 

Pig  Embryo  of  20  mm.     Study  of  Sections,   311 

Transverse  Sections 311 

Sagittal  Section, 322 

Frontal  Sections  of  the  Head, 324 

Pig  Embryo  of  24  mm.     Study  of  Sections,   330 

CHAPTER  VII. — STUDY  OF  THE  UTERUS  AND  THE  FETAL  APPENDAGES  OF  MAN, 339 

Histology  of  the  Uterus,       339 

Menstruation,    339 

The  Pregnant  Uterus:  the  Two  Stages,    341 

Human  Uterus  Three  Months  Pregnant, 343 

Human  Uterus  Seven  Months  Pregnant, 345 

Decid.ua  Vera  of  the  First  Stage  in  Section, 346 

Decidua  Reflexa  of  the  First  Stage, 349 

Decidua  Vera  and  Chorion  Laeve  of  the  Second  Stage, 350 

The  Placenta  in  Situ, 352 

Decidua  Serotina  at  Seven  Months, 357 

The  Human  Placenta,      359 

Histology  of  the  Human  Chorion,  363 

The  Chorion  with  Trophoderm, 364 

The  Chorionic  Villi,    367 

The  Structure  of  the  Amnion,    , 370 

The  Umbilical  Cord, 372 

The  Structure  of  the  Human  Yolk-sac, 375 


xii  CONTENTS. 

PAGE 

CHAPTER  VIII.— METHODS, 377 

Measuring  Length  of  Embryos, 377 

Dissection  of  Embryos, 377 

Methods  of  Hardening  and  Preserving, 377 

Preservation  in  Alcohol, 380 

Directions  for  Imbedding  Specimens  to  be  Microtomed, 380 

Method  of  Mounting  Paraffin  Sections,  381 

Methods  of  Staining, 381 

Methods  of  Reconstruction, 385 

Directions  for  Orienting  Serial  Sections  of  Embryos,  388 

Microtomes, 389 

INDEX, 393 


TEXT-BOOK  OF  EMBRYOLOGY. 


CHAPTER  I. 
GENERAL    CONCEPTIONS. 

The  student  of  embryology  should  start  with  as  clear  and  definite  a  concep- 
tion as  possible  of  what  he  is  to  gain  from  his  pursuit  of  that  science.  If  he  is  a 
student  of  biology  or  of  zoology,  he  must  appreciate  that  knowledge  of  the  laws 
of  development  is  an  indispensable  part  of  what  he  must  master  in  order  to 
understand  those  sciences.  He  must  appreciate  that  it  is  from  the  studies  of  the 
embryologist  that  are  derived  our  conceptions  of  the  nature  of  sex,  of  heredity,  of 
variation,  of  differentiation,  and  many  of  our  most  important  notions  concerning 
evolution,  both  of  the  individual  and  of  the  race.  He  will  learn  further  that  the 
embryo  illustrates  to  him  with  particular  clearness  the  fundamental  principles 
of  morphology.  If  he  be  a  medical  student,  he  will  find  in  embryology  first 
of  all  the  clue  to  the  intelligent  comprehension  of  the  anatomy  of  the  adult,  a 
comprehension  which  he  can  obtain  in  no  other  way,  but  he  will  also  gain  much 
knowledge  of  direct  practical  value  as  to  the  embryo  and  as  to  the  conditions  in 
the  adult,  acquaintance  with  which  is  invaluable  in  medical  practice.  And. 
finally,  he  will  find  that  it  throws  a  vast  light  on  pathology,  both  upon  the  prob- 
lems of  malformations  and  monstrosities,  and  also  upon  the  whole  question  of 
pathological  change  in  the  tissues. 

The  best  study  of  embryology,  therefore,  is  that  which  continually  passes 
beyond  the  direct  observations  to  the  conceptions  which  they  justify  and  which 
underlie  many  important  branches  of  science  which  are  related  to,  and  in  a  large 
part  dependent  upon,  embryology. 

The  student  ought  to  strive,  accordingly,  to  pass  from  the  direct  observation 
of  the  specimen  to  the  generalizations,  and  accustom  himself  to  regard  always 
each  special  preparation,  which  may  be  submitted,  to  his  observation,  as  an  illus- 
tration of  some  general  principle.  To  facilitate  his  reaching  this  result  this 
chapter  offers  a  digest  of  some  of  the  more  important  generalizations  and  funda- 
mental laws  of  embryology. 


•2  GENERAL  CONCEPTIONS. 

The  Segmented  Animals.    Metamerism. 

All  vertebrates  and  certain  invertebrates  haye_-their  bodies  divided  into  a 
series  of  parts,  which  begins  in  the  region  of  the  head  and  extends  to  the  caudal 
end  of  the  body.  These  parts  are  called  segments  or  metameres.  In  man  each 
vertebra  corresponds  to  one  segment.  In  the  earthworm  and  other  annelids 
the  segments  are  plainly  marked  on  the  outer  surface  of  the  body.  Since  all 
segmented  animals  are  bilaterally  symmetrical,  each  segment  is  bilaterally  sym- 
metrical and  may  therefore  be  described  as  a  paired  structure.  Embryology 
has  demonstrated  that  each  segment  arises  as  a  pair  of  masses  (somites]  situated 
between  the  digestive  canal  and  the  outer  surface  of  the  body.  Each  pair  of 
masses  is  formed  from  the  middle  germ-layer  (mesoderm)  exclusively  (compare  pages 
84-85)  and  is  known  as  "a  primitive  segment."  The  mesodermic  somites  are 
to  be  considered  the  essential  primary  morphological  segments,  and  in  the  course 
of  the  development  of  the  individual  they  produce  adult  metameric  structures, 
among  which  may  be  muscles,  skeletal  elements,  excretory  organs,  etc.,  the  whole 
history  of  which  depends  upon  their  segmental  origin.  The  nervous  system  and 
to  a  considerable  extent  the  blood-vessels  exhibit  a  segmental  arrangement,  which 
is  usually  regarded  as  a  secondary  correlation  of  these  structures  with  the  primary 
mesodermic  metamerism.  The  spinal  nerves  exemplify  the  correlation.  In  brief, 
where  the  segmental  organization  exists,  it  dominates  the  anatomy  alike  of  the  embryo 
and  the  adult;  therefore  the  student  of  embryology  should  pay  special  attention 
to  the  segmentation  of  the  body  in  all  its  chief  stages. 

In  regard  to  the  bodily  segmentation  two  general  observations  may  be  made : 
First,  the  development^  of  segments  begins  at  the-  cephalic  end  and  progresses 
tailward;  hence  so  long  as  the  development  of  segments  continues  various  stages 
of  their  differentiation  may  be  found  in  a  single  embryo,  the  more  advanced 
stages  being  always  cephalad  from  the  less  advanced.  Second,  there  is  a  funda- 
mental difference  between  the  metamerism  of  vertebrates  and  that  of  annelids 
and  many  other  invertebrates,  which  consists  in  the  unlike  extent  of  the  segments; 
for  the  primitive  segments  of  vertebrates  are  confined  to  the  dorsal  region  of  the 
body,  while  in  the  other  forms  the  segmentation  extends  from  the  start  through  the 
dorsal  and  ventral  regions  both.  It  is  probable  that  the  segments  of  vertebrates 
are  homologous  only  with  the  dorsal  part  of  the  segments  of  annelids. 

The  Vertebrate  Type  of  Structure. 

When  one  traces  the  course  of  development  of  any  vertebrate,  one  finds, 
speaking  in  general  terms,  that  the  fundamental  characteristics,  which  are  more 
or  less  common  to  all  vertebrates,  are  those  which  first  appear.  Later,  there 
come  in  the  secondary  characteristics  which  distinguish  one  class  from  another, 
and  still  later  the  subordinate  characteristics  by  which  the  smaller  subdivisions 
of  the  vertebrate  type  become  differentiated  one  from  another.  This  statement, 
however,  is  correct  only  if  we  add  to  it  certain  indispensable  limitations.  Every 
embryo  at  every  stage  of  its  development  is  an  individual  of  the  particular  genus 


THE  VERTEBRATE  TYPE  OF  STRUCTURE.  3 

and  species  to  which  it  belongs.  It  has  at  every  stage  peculiarities  which  dis- 
tinguish it  from  every  other  species.  The  embryos  of  allied  forms  resemble 
one  another  more  closely  than  do  the  embryos  of  forms  which  are  only  distantly 
related  to  one  another.  The  specific  qualities  of  an  embryo  are,  however,  more 
difficult  to  recognize  than  those  of  the  -adult,  and  the  student  will  be  far  more 
impressed  by  the  resemblances  between  embryos  than  by  their  differences.  It 
is  owing  to  this  very  fact  that  the  distinctive  peculiarities  of  the  species  are  not 
accentuated  in  the  embryo.  We  are  able  to  derive  from  the  embryos  themselves 
a  series  of  conceptions  which  render  it  comparatively  easy  to  perceive  •  the  domi- 
nant morphological  features  of  the  vertebrate  type. 

It  will  be  convenient  to  put  down  six  fundamental  characteristics  of  the  ver- 
tebrate type  as  the  most  important,  and  to  add  to  these  six  others  which  are 
also  fundamental,  but  perhaps  less  distinctive.  This  enumeration  is  necessarily 
arbitrary,  and  can  serve  only  to  facilitate  the  work  of  the  student.  When  his 
knowledge  deepens,  he  will  be  able  to  free  himself  from  the  limitations  which 
such  a  numerical  classification  may  have  put  on  his  understanding  of  the  matter. 

A.  The  six  most  important  characteristics  are: 

/  i.  The  pharynx  and   pharyngeal  structures    (gill-clefts,  nerves,  aortic   arches, 

heart). 
./  2.  The  notochord  or  structural  axis. 

3.  Tubular  central   nervous   system. 

4.  Limbs. 

5.  Position   of   mouth. 

6.  Division  of  the  ccelom  into: 

(a)  dorsal  segmented  part  or  cavities  of  the  somites. 

(b)  ventral    unsegmented    part    (splanchnocele),    which    is    subdivided    by 
the   septum   transversum   into   a   thoracic   and   an   abdominal    portion. 

B.  Other  fundamental  but  less  distinctive  characteristics  are: 

7.  Stomach,   intestine,  and  mesentery. 

8.  Position  of  liver,  and  its  relation  to  veins. 

9.  Wolffian  tubules  and   ovotestis    (  =  urogenital  ridge). 

10.  Urogenital   ducts    (Wolffian   and   Miillerian). 

11.  Special  sense-organs    (nose,   eye,   and  ear). 

12.  Hypophysis. 

The  pig  embryo  illustrates  all  these  characteristics,  and  we  shall  study  the 
ways  in  which  the  typical  mammalian  modifications  of  the  type  are  gradually 
evolved. 

Let  us  now  pass  in  review  these  twelve  characteristics. 

i.  The  pharynx  is  the  cephalic  portion  of  the  digestive  canal,  and  it  acquires 
in  all  vertebrates  a  somewhat  complicated  structure.  This  complication  de- 
pends primarily  upon  a  series  of  lateral  outgrowths  from  the  pharynx  which  are 
known  by  the  name  of  gill-pouches.  They  are  symmetrically  arranged  and  there- 


GENERAL  CONCEPTIONS. 


fore  form  pairs.  They  are  designated  by  numbers,  the  pouch  which  lies  nearest 
to  the  mouth  being  called  the  first,  the  next  the  second,  and  so  on.  Among 
the  lower  vertebrates  the  number  of  these  gill-pouches  varies  from  five  to 
perhaps  nine  pairs.  In  mammals  there  are  always  four  distinct  pairs.  In 
aquatic  vertebrates  the  pouches  acquire  each  an  opening  to  the  exterior  at  the 
side  of  the  neck,  and  are  then  designated  as  gill-clefts  or  branchial  clefts.  We 
find  that  the  position  of  the  clefts  determines  the  distribution  of  a  series  of  the 
most  important  of  the  cephalic  nerves  and  the  primitive  distribution  of  the  branches 
of  the  aorta  and  of  certain  important  muscles,  hence  the  morphological  features 
of  the  pharynx  have  a  profound  influence  upon  the  entire  anatomy  of  the  body 
in  that  region.  No  similar  pouches  are  formed  from  any  other  part  of  the  di- 
gestive canal.  The  pharynx  also  gives  rise  to  the  thyroid  gland,  the  anlage  of 
which  starts  as  an  outgrowth  from  the  median  ventral  side  of  the  pharynx.  The 
entoderm  of  the  third  pair  of  gill-pouches  produces  the  anlages  of  thymus  glands, 
and  that  of  the  fourth  pair  the  anlages  of  the  parathyroids. 

2.  The  notochord  is  a  rod  of  cells  which   extends    nearly  the   entire  length   of 
the  embryo.     It   lies   in  the   median   plane,   a   little   below   the   ventral   edge   of   the 
central    nervous    system.     Its    cephalic    termination    is    always    in    the    neighborhood 
of   the   pituitary   body.     It   may   be   considered   the   primitive   structural   axis   of   the 
vertebrates.     There  are  vertebrates  in  which  it  is  the  only  structural  axis  ever  pro- 
duced,  but    in    the    great    majority    of    vertebrates    there    is    developed    around    the 
notochord   a   series   of    skeletal   elements   which   we    know   as    vertebrae,    and   which 
make   a   new   structural   axis    in   these    forms.     The   notochord    in    these    animals    is 
found    to   run   through,  the   bodies   of   the    vertebrae.     The    notochord    diminishes    in 
size    as    we    ascend    the  vertebrate    series.     It    is    of    very    considerable    diameter    in 
the   lowest   fishes,   smaller   in   amphibia   and   reptiles,    and  smallest   of   all    in   mam- 
mals.    In    the    lower    forms    it    persists    throughout    life    as    a    continuous    rod.     In 
the  higher  forms  it  tends  to   become  attenuated   in  the  vertebral,  expanded  in  the 
intervertebral,    regions,    and    in    adult    mammals    persists    only    as    a /series    of    dis- 
connected thickenings   (nuclei  pulposi}   between  the  vertebras. 

3.  The  tubular  central  nervous  system.     This   is   found   in   vertebrates   only,   or 
in    animals    which    are    closely    related    to    vertebrates,    so    closely    that    b.y    many 
naturalists  they  are  included  in  the  same  subkingdom.     The  hollow  nervous  system 
is    enlarged    in    the    region    of    the    head,    the    enlargement    constituting    the    brain. 
The  rest  of  it  is  of  smaller  size  and  constitutes  the  spinal  cord.     That  the  brain 
and  spinal   cord   form   the   wall   of   a   tube   is   one .  of   the   fundamental   conceptions 
of  anatomy. 

4-  The  limbs.  There  are  two  pairs,  which  are  lateral  extensions  of  the  sur- 
face of  the  body  and  acquire  in  their  interior  a  skeleton  by  which  they  are 
supported  and  muscles  by  which  they  are  moved.  No  structures  in  any  invertebrate 
animal  are  known  to  be  homologous  with  vertebrate  limbs. 

5.  The   position  of  the   mouth.     The   typical   invertebrate   mouth   is   surrounded 


THE  VERTEBRATE  TYPE  OF  STRUCTURE.  5 

by  the  nervous  system.  For  instance,  in  insects  or  in  the  jointed  worms  (annelids) 
there  is  a  brain,  so  called,  above  the  mouth,  and  a  strand  of  nervous  tissue 
running  down  on  either  side  of  the  body  past  the  mouth  to  join  the  ganglion 
on  the  lower  side,  thus  completing  a  circumoral  ring  of  nervous  material  through 
which  the  oesophagus  passes.  In  vertebrates,  on  the  other  hand,  the  mouth  is 
not  enclosed  by  any  cesophageal  ring,  and  the  entire  nervous  system  is  on  one 
side  of  the  body  and  dorsal  to  the  mouth. 

6.  The    division    of    the    primitive    body-cavity.     The    body-cavity    in    the    em- 
bryo is  known  by  the  comprehensive  name  of  the  ccelom.     It  will  not  be  possible 
to  acquire  a  clear  idea   of   its   division   until  the  embryos  are   actually  studied.     It 
forms    many  parts.     Of  these  there  are  two  dorsal  series,  one  on  each    side  of   the 
central  nervous  system,  which  form  cavities  of  what  we  designate  as  the  somites  of 
the    body.     There    are    also    two    large    ventral    divisions    which    extend    from    the 
region  of    the  head  to  that  of    the  future  pelvis,  one  division  for  each  side  of   the 
body.     These    two    large    parts    are    not    divided    into    segments    at    all,    though    the 
cavities    of    all    of    the    segments    are    primitively    connected    with    these    two    main 
divisions.     Comparatively    early    in^J;he    development    the    two    main    cavities    be- 
come connected  with  one  another,\so  as  to  constitute  a  single  cavity  to  which   we 
apply    the     name    of     splanchnocele.      The    splanchnocele     surrounds    the    heart    of 
the  embryo,  where  we  recognize  it  as  the  pericardial  cavity,  and  it  extends  through 
the    future    abdominal    region,    where    we    recognize    it    as    the    abdominal    cavity. 
The    pericardial    and    abdominal    regions    of    the    cavity    are    separated    from    one 
another  in    the    embryo    by    a    broad    transverse    partition    which    bears    the    name 
of   septum   transversum.     This  septum    in    mammals    becomes    in    the    adult   the   dia- 
phragm.    It    is    one    of    the    most    striking    of    all    the    morphological  peculiarities 
by  which  vertebrates  are  distinguished  from  invertebrates. 

7.  The    stomach,     intestine,     and     mesentery.     The     division    of     the    digestive 
tract    of    vertebrates    into    two    fundamental    parts,    stomach    and    intestine,    is    very 
characteristic.     The    stomach    is    not    only  an  enlargement    of    the    digestive    canal, 
but   also   may 'be   distinguished   from   the   intestine   by   its   developing   glands,   which 
are    specific    to    it    and    unlike    those  of    the   intestine  proper.     The   elongated  oeso- 
phagus   occurs    in   the    higher   vertebrates   only,  and   is  •  not   a   general   characteristic 
of    the    subkingd<3m.     The    mesentery   by   which    the   intestine   is   suspended   to   the 
dorsal  wall   of  the  abdomen   is   the   survival  of   the  original   partition  by   which  the 
two    halves    of  the    splanchnocele    were    separated    from    one    another.     The    cavities 
in   the   abdominal   region   come   into   communication   with   one   another   by   the   very 
early   disappearance    of    the   partition    on  the   ventral   side   of    the   intestine.     But   it 
should   be   noted   at  once   that  a   portion   of  this   primitive   ventral   partition,   or,  as 
we  may  call  it,   ventral  mesentery,   persists   permanently  in   relation   to  the   position 
of  the  liver. 

8.  The    position    of    the    liver.     The    primitive    large  veins    of    the    embryo    pass 
through   the   septum   transversum,  and   it   is   by  intercrescence  with   these  veins,  and 


6  GENERAL  CONCEPTIONS. 

as   an   appendage   to   the   septum   itself,    that   the   liver   is  developed,    although    it   is 
produced  by  a  special  local  growth  of  the  digestive  canal. 

9.  The   urogenital   ridge.     Out   of   a  part   of   the   primitive   segments   there   are 
developed    excretory    organs,    and    these,    as    they  increase    in    size,    form    two    pro- 
tuberances   on   the   dorsal   side   of    the   splanchnocele.      Each    protuberance   is   what 
we    know   as   the   urogenital   ridge,   so   named,   first,    on   account    of   its    form;    and, 
secondly,  on   account    of    its    producing   not    only    the    excretory    organs    proper,  but 
also  the  genital  glands. 

10.  The    urogenital    ducts.     There  is    primitively    a    single    duct    for    each    uro- 
genital ridge.     This  duct  is  commonly  known  as  the  Wolffian  duct.     A  little  later  in 
the   history   of   the   embryo   there   appears   a   second   canal  known  as  the  Mullerian 
duct,  which  is  closely  parallel  to  the  first,  but  which  has  no  connection  with  any  of  the 
excretory  apparatus,  and  is  destined  to  serve  later  as  the  female  genital  duct.     In  no 
invertebrate  have  we  found  anything  certainly  homologous  with  these  two  ducts. 

11.  Special    sense-organs.     These    are    the    olfactory,    the    visual,    the  so-called 
auditory   organs,    and    the   organs    of   the    lateral   line.     We    have    to    use    the    term 
"so-called"  in  speaking  of  the  auditory  organ  because  we  now  know  that  the  ear 
in  the  lower  vertebrates  is  not  an  organ  of  hearing,    but  an  organ  of  balancing  or 
orientation,    and   it   is   only   in   the    higher   vertebrates   that   there    is    added    to    this 
primitive    function    that   of   audition   proper.     It   seems    not    improbable    that    many 
invertebrate  animals   have  sense-organs  which   are   homologous  with  those  of   verte- 
brates.    Nevertheless,  in  the  vertebrate  type  there  are  many  peculiarities  which  are 
distinctive,  and  these  we  shall  best  learn  from  a  'study  of  the  actual  development. 
The    sense-organs  of    the  lateral  line  are   highly  developed   important   structures   in 
the   Ichthyopsida,    but   apparently   are    not   represented    in    mammals    at    any   stage 
of  their  development. 

12.  The    hypophysis.     The    hypophysis    is    the    embryological    name   applied  to 
the   structure   which   we   know    in   the   adult    as   the   anterior   lobe   of   the  pituitary 
body.     The  posterior  or  infundibular    lobe  is  a    portion    of    the    brain,   but  the  an- 
terior   lobe    is    an    outgrowth    from    the    cavity  of    the  mouth  of  the  embryo.     Com- 
paratively   early    in    the    development    of    the    individual    this    outgrowth    becomes 
entirely   separated   from   the   mouth-cavity    (from   the   walls   of   which   it   arose),    and 
forms  a   closed    vesicle.     It    exists    in    every    known    vertebrate    animal,    has    been 
much  studied,  but  still  remains  an  organ  the  significance  of  which   we  cannot  ex- 
plain.    Its    absolute    persistency    and    the    uniformity    of    its    development    indicate 
that  it  is  an  organ  of  importance,  but  beyond  that  we  can  hardly  go. 

To  these  conceptions,  the  student  should  add  the  following  comprehensive 
morphological  notions:  The  mammalian  body  may  be  defined  as  two  tubes  of 
epithelium,  one  inside  the  other;  the  outer  tube  (epidermal  or  ectodermal)  is  very 
irregular  in  its  form;  the  inner  tube  (entodermal)  is  much  smaller  in  diameter, 
but  much  longer  than  the  outer  and  has  a  number  of  branches  (lung,  pancreas, 
etc.),  and  is  placed  within  the  ectodermal  tube.  Between  these  two  tubes  is  the 


PRINCIPAL  MODIFICATIONS  OF  THE  VERTEBRATE  TYPE.   .  7 

very  bulky  mesoderm,  which  is  divided  by  large  cavities  (abdominal  and  thoracic) 
into  two  main  layers,  one  of  which  is  closely  associated  with  the  epidermis  and 
forms  the  body  wall,  the  somatopleure  of  embryologists;  the  other  joins  with  the 
entoderm  to  complete  the  walls  of  the  splanchnic  viscera,  and  constitutes  the 
splanchnopleure  of  embryologists.  The  mesoderm  is  permeated  by  two  sets  of 
cavities:  (i)  the  heart  and  blood-vessels;  (2)  the  lymphatic  system.  It  is  also 
differentiated  into  numerous  tissues  (muscles,  tendon,  bone,  etc.) ,  organs  and  the 
internal  parts  of  the  urogenital  system.  The  nervous  system,  although  developed 
from  the  ectoderm,  is  found  separated  from  its  site  of  origin,  and  completely  en- 
cased in  mesoderm. 

The  Principal  Modifications  of  the  Vertebrate  Type. 

Our  knowledge  of  human  development  being  at  the  present  time  incom- 
plete, it  is  often  necessary  to  supplement  that  knowledge  by  reference  to  facts 
of  observation  on  the  development  of  various  vertebrates.  Indeed,  the  best  study 
of  human  embryology  includes  more  or  less  comparative  work.  We  shall,  therefore, 
find  frequent  occasion  to  refer  to  the  development  of  many  vertebrate  types. 
Accordingly,  in  this  section  there  are  given  definitions  of  the  principal  subdivisions 
of  the  vertebrates  to  which  we  shall  have  occasion  to  refer. 

From  an  embryological  standpoint,  vertebrates  may  be  separated  into  two 
main  divisions,  which  are  commonly  designated  as  the  Amniota  and  Anamniota, 
distinguished  by  the  presence  or  absence  of  the  amnion,  the  amnion  being  a  thin 
membrane,  which  immediately  surrounds  the  embryo  in  the  higher  forms.  It 
occurs  in  reptiles,  birds,  and  mammals,  which  together  constitute  the  Amniota. 
It  is  absent  in  the  fishes  and  amphibians,  which  therefore  constitute  the  Anamniota. 
These  two  divisions  are  also  distinguished  by  other  peculiarities.  The  higher  forms 
referred  to  all  have  the  organ  known  as  the  allantois,  an  appendage  of  the  embryo, 
which  is  lacking  in  the  lower  forms.  The  comparative  anatomist  finds  many 
points  of  resemblance  between  the  various  classes  of  fishes,  on  the  one  hand,  and 
the  amphibia,  on  the  other,  and  indicates  this  relationship  by  the  use  of  the  term 
Ichthyopsida,  which  means  "fish-like."  In  our  present  classification  the  term 
Ichthyopsida  is  synonymous  with  Anamniota.  The  comparative  anatomist  further 
recognizes  a  close  relationship  between  birds  and  reptiles,  and  puts  these  together 
under  the  common  designation  of  Sauropsida,  or  "reptile-like." 

As  regards  the  fishes,  many  classifications  are  more  or  less  in  vogue  at  the 
present  time.  For  the  purposes  of  this  book,  the  following  names  for  the  classes  have 
been  adopted  as  names  generally  understood  and  sufficiently  exact  to  meet  our 
needs:  The  lowest  fishes  are  the  hag-fishes  and  lampreys,  constituting  the  group 
of  Marsipobranchs.  Next  comes  the  group  comprising  the  sturgeon  and  its  allies, 
for  which  we  have  retained  the  old  term  of  Ganoids.  To  these  fishes  the  central 
position  in  the  system  must  be  assigned,  and  it  is  probable  that  the  higher  fishes 
are  more  or  less  directly  descended  from  Ganoid-like  forms.  They  fall  into  three 


GENERAL  CONCEPTIONS. 

further  classes,  of  which  the  largest  and  most  varied  is  that  of  the  bony  fishes, 
or  Teleosts.  Another  class,  known  as  the  Elasmobranchs,  comprises  the  sharks, 
skates,  rays,  and  electric  fishes.  The  last  class  is  known  as  the  Dipnoi,  or  lung 
fishes,  which  comprise  only  three  living  forms,  the  Ceratodus,  living  in  Australia, 
the  Protopterus  in  Africa,  and  the  Lepidosiren  in  South  America. 

The  amphibia  are  divided  into  two  classes,  the  Urodela,  of  which  the  newts 
and  salamanders  are  familiar  examples,  and  the  Anura,  of  which  the  frogs  and 
the  toads  are  -the  best  known  representatives.  The  two  types  are  easily  distin- 
guished by  the  presence  or  absence,  respectively,  of  the  tail  in  the  adult. 

As  to  the  reptiles,  it  is  unnecessary  to  consider  their  classification,  as  we  shall 
not  have  much  occasion  to  refer  to  them,  our  knowledge  of  their  embryology 
being  very  fragmentary  at  the  present  time,  save  for  a  rather  extended  series 
of  observations  on  the  development  of  lizards.  As  regards  birds,  it  may  be  noted 
that  embryologists  have  worked  chiefly  upon  the  chick,  which  iias  been  for  a 
century  the  classic  object  of  embryological  study.  There  are  comparatively  few 
observations  on  the  development  of  other  species  of  birds. 

Mammals  are  divided  into  three  principal  classes.  Of  these,  the  lowest  is 
that  of  the  Monotremes,  of  which  the  only  living  representatives  are  found  in 
Australia  and  neighboring  islands,  a  very  few  species  concerning  the  develop- 
ment of  which  very  little  is  as  yet  known,  but  which  are  of  importance,  as  they 
resemble  in  certain  respects  the  reptiles  and  assist  us  in  drawing  comparisons 
between  the  reptilian  and  the  mammalian  types.  Of  this  class,  the  Australian 
duck-bill  may  be  mentioned  as  typical. 

The  second  class  is  that  of  the  Marsupials,  familiar  to  us  in  America 
through  the  common  opossum.  In  Australia  there  are  many  genera  and  species 
of  marsupials. 

The  third  class  comprises  the  majority  of  well-known  mammals,  and  may 
be  termed  the  Placentalia,  and,  for  embryological  purposes,  it  is  convenient  to 
consider  the  Placentalia  as  forming  two  principal  subclasses,  the  animals  with 
claws  and  the  animals  with  hoofs,  the  Unguiculates  and  the  Ungulates.  Of  the 
Unguiculates,  we  shall  have  occasion  to  refer  to  the  Insect-Ivor  a,  of  which  the  mole 
may  serve  as  a  type;  the  Cheiroptera,  or  bats;  the  Rodents,  including  the  rats. 
guinea-pigs,  rabbits,  etc.;  the  Carnivora,  cats,  dogs,  and  allied  animals;  and,  finally, 
the  Primates,  which  include  the  lemurs,  monkeys,  apes,  and  man. 

Of  the  Ungulates,  we  shall  have  occasion  to  refer  chiefly  to  the  pig  and  the 
sheep.  The  following  table  presents  in  their  proper  order  those  animals  which 
we  shall  have  occasion  to  consider. 

Annelida 
Atriozoa 

Tunicata  (Ascidia) 

Cephalochorda 
Amphioxus 


DEFINITION  OF  AN  LACE.  9 

Vertebrata 

Anamniota  (Anallantoidea) 
Ichthyopsida 
Pisces 

Marsipobranchia  (lampreys,  etc.) 
Ganoidea  (sturgeon,  etc.) 
Teleostea  (bony  fishes) 
Elasmobranchia  (sharks,  skates,  etc.) 
Dipnoi  (lung-fishes) 
Amphibia 

Urodela  (newts,  salamanders,  et:.) 
Ansura  (frogs,  toads) 
Amniota  (Allantoidea) 
Sauropsida 

Reptilia  (lizards,  crocodiles,  snakes,  turtles,  etc.) 
Aves 
Mammalia 

Montotremata  (duck-bill,  etc.) 
Marsupialia  (opossum,  kangaroo,  etc.) 
Placentalia 

Unguiculate  series 

Insectivora  (moles,  etc.) 
Cheiroptera  (bats) 

Rodentia  (rats,  rabbits,  guinea-pigs,  etc.) 
Carnivora  (cats,  dogs,  etc.) 
Primata  (lemurs,  monkeys,  apes,  man) 
Ungulate  series 
Ungulata 

Artiodactyla  (even-toed)  (cattle,  sheep,  pig,  deer,  etc.) 
Perissodactyla  (uneven-toed)   (horse,  rhinoceros,  etc.) 

Of  the  invertebrate  animals  there  will  be  little  to  be  said.  There  are  two 
types  of  invertebrates  which  show  relationship  in  their  structure  to  true  verte- 
brates. One  of  these  is  the  class  of  jointed  worms,  or  Annelids;  the  other  is  the 
class  of  Atriozoa,  which  comprises  the  subdivisions  of  Tunicata  and  of  the  Cepha- 
lochorda.  All  of  our  observations  on  the  development  of  this  last  type  are  based 
on  the  one  genus,  Amphioxus,  which  will  therefore  be  the  name  which,  we  shall 
use  whenever  we  have  to  refer  to  these  animals. 

Definition  of  Anlage. 

There  will  be  frequent  occasion  to  use  this  word  in  a  strictly  technical  sense. 
It  has  been  adopted  from  the  German,  as  there  is  no  satisfactory  English  equiva- 
lent for  it.  The  French  use  the  word  "ebauche"  and  the  Italians  " abozzo." 
Primordium  has  been  proposed  as  the  Latin  equivalent  and  is  used  by  a  few 
American  authors,  but  anlage  is  generally  employed  by  both  American  and  English 
writers.  "Anlage''  may  be  denned  as  follows:  The  first  accumulation  of  cells 
in  the  developing  embryo  recognizable  as  the  commencement  of  a  structure,  organ, 
or  part. 


10  GENERAL  CONCEPTIONS. 

A  Summary  of  Embryological  Development. 

The  following  summary  applies  to  what  is  known  of  vertebrates  only.  It  I 
would  require  some  modifications  to  be  applicable  to  the  whole  animal  kingdom. 
Each  individual  arises  froni  a  single  cell  which  is  termed  the  impregnated  or  fer- 
tilized ovum.  From  this  all  embryological  study  starts.  The  fertilized  ovum 
has  its  earlier  history,  since  it  is  the  product  of  the  fusion  of  two  sexual  elements. 
It  is  a  living  cell,  and  therefore  contains  protoplasm  and  nucleus.  It  is  also 
furnished  with  a  certain  amount  of  material  known  as  yolk,  which  exists  in 
the  form  of  separate  granules  imbedded  in  the  protoplasm.  This__y_olk  js  the 
reserve  food  material,  and  by  the.  assimilation  thereof  the  protoplasm  of  the 
ovum  can  grow. 

The  first  step  in  the  development  r,  tht  repeated  division  of  the  original  cell 
so  that  there  is  produced  an  increasing  number-  of  cells.  The  earlier  stages  of  this 
cell  multiplication  are  designated  as  the  segmentation  of  the  ovum.  This  name  is 
due  to  the  fact  that  the  process  was 'first  observed  in  the  eggs  of  amphibia  in  the 
early  part  of  the  last  century,  before  the  cell  doctrine  had  been  established.  In 
default  of  a  better  name,  the  separate  cells  into  which  the  ovum  divided  were 
called  segments,  for  it  was,  of  course,  not  known  that  they  were  cells.  Although 
this  term  is  no  longer  appropriate,  it  is  still  universally  used  because  of  its  con- 
venience. There  are  two  principal  types  of  ovum  known:  in  one  type  wre  find 
only  a  small  amount  of  yolk  material;  in  the  other  a  very  large  amount.  There 
are  ova  known  intermediate  between  these  two  types.  When  the  ovum  is  of  the 
first  type  ,  the  whole  of  it  undergoes  segmentation  at  once,  and  to  such  an  ovum 
the  term  holoblastic  is  applied.  In  the  second  type,  on  the  contrary,  we  find  that 
the  protoplasm  tends  to  accumulate  at  one  pole  of  the  cell  and  the  yolk  granules 
at  the  other.  The  protoplasmic  portion  exhibits  a  far  more  active  cell  division 
than  the  yolk-bearing  portion,  so  that  the  segmentation  seems  to  take  place  exclu- 
sively around  one  pole  or  part  of  the  ovum,  which  is,  therefore,  designated  as 
\meroblastic.  After  the  segmentation  of  the  ovum  the  multiplication  of  the  cells 
continues,  and  they  gradually  arrange  themselves  in  such  a  manner  as  to  form 
three  distinct  sheets  or  laminae,  which  are  named  "germ-layers.'"  These  layers 
are  designated:  the  outermost  as  Ectoderm,  the  innermost  as  Entoderm,  and  the 
middle  as  Mesoderm*  From  an  embryological  point  of  view  the  importance 
of  these  three  primitive  germ-layers  cannot  be  over-emphasized.  The  principal 
occupation  of  the  student  will  be  to  familiarize  himself  with  the  appearance 
of  these  layers  and  the  modifications  which  they  undergo,  and  the  adult  tissues 
which  are  produced  from  them.  They  dominate  every  phase  of  development, 
the  form  of  every  organ,  the  production  of  every  tissue.  Their  importance  is 
so  great  that  embryology  might  almost  be  defined  as  the  science  of  germ-layers. 

*  Some  English  and  occasionally  Continental  authors  use  other  terms  for  the  germ-layers,  namely,  for 
ectoderm,  epiblast;  for  entoderm,  hypoblast;  for  mesoderm,  mesoblast.  I  have  preferred  to  maintain  the  older 
terms  which  have  been  in  almost  universal  use  for  a  century. 


CYTOMORPHOSIS.  11 

The  primitive  germ-layers  consist  of  very  simple  cells,  and  are  themselves 
at  first  extremely  simple  in  their  organization.  The  majority  of  the  cells  which 
they  contain  undergo  a  greater  or  less  degree  of  modification  as  development 
progresses.  This  modification  is  termed  differentiation,  and  is  more  fully  con- 
sidered in  our  next  section,  on  Cytomorphosis.  It  is  probable,  however,  that  a 
certain  number  of  the  cells  very  early  in  the  development  are  set  apart,  preserving 
the  primitive  character  of  their  protoplasm  and  taking  no  share  in  the  formation 
of  the  tissues  of  the  body.  These  cells,  comparatively  unmodified,  are  known  as 
the  germ-cells;  compare  page  25  and  the  section  on  Heredity.  As  the  remaining 
cells  form  part  of  the  body  of  the  individual,  they  may  be  designated  as  somatic 
cells.  Besides  the  process  of  differentiation  of  the  cells,  we  find  that  the  production 
of  organs  -is  largely  dependent  upon  the  unequal  growth  of  the  germ-layers,  one 
part  growing  rapidly,  another  more  slowly,  so  that  the  layers  acquire,  as  the 
embryo  develops,  a  more  or  less  complicated  form,  owing  to  the  folding  of  the 
layers.  The  "general  principles  which  govern  these  important  developments  are 
considered  in  the  section  upon  the  Relations  of  Surface  to  Mass. 

Cytomorphosis. 

This  term  is  used  to  designate  comprehensively  all  the  structural  modifica- 
tions which  cells  or  successive  generations  of  cells  may  undergo,  from  the  earliest 
undifferentiated  stage  to  their  final  destruction.  It  will  be  convenient,  though 
somewhat  arbitrary,  to  distinguish  four  fundamental  successive  stages  of  cyto- 
morphosis.  These  stages  are  (i)  the  undifferentiated  stage;  (2)  the  stage  of 
progressive  differentiation,  which  itself  often  comprises  many  successive  stages; 

(3)  the  regressive  stage  or   that    during    which    degeneration    or    necrobiosis^  occurs; 

(4)  the  stage  of  the  removal  of  the  dead  material. 

In  the  various  parts  of  the  body  we  find  these  stages  to  succeed  one  another 
at  varying  rates,  and  there  are  always  to  be  found  in  every  living  vertebrate 
body  a  considerable  number  of  cells  which  have  passed  through  only  a  certain 
differentiation  and  do  not  present  any  of  the  phenomena  of  degeneration  or  of  death. 
On  the  other  hand,  there  are  cells  at  every  epoch  of  life  after  an  early  brief  em- 
bryonic period  which  degenerate  and  die  off,  although  the  life  of  the  individual 
is  uninterrupted.  At  any  given  moment  the  body  consists  of  cells  which  have 
made  unequal  progress  through  the  cytomorphic  cycle. 

i.  The  Undifferentiated  Stage. — A  fertilized  ovum  is  an  undifferentiated  being, 
although  it  has  a  very  complex  organization.  As  it  has  only  one  nucleus  there 
can  be  no  variety  of  nuclei.  The  term  "undifferentiated"  therefore  applies  es- 
pecially to  the  protoplasm,  which  never  has  any  special  structures  or  formed 
parts,  such  as  occur  in  the  tissues  and  cells  of  the  adult.  It  is,  however,  not  uni- 
form, but  in  many  ova  has  distinct  regional  differences,  which  so  far  as  hitherto 
noted  depend  upon  peculiarities  in  the  masses  and  strands  of  protoplasm,  and 
upon  the  distribution  of  the  yolk  granules,  of  which  there  may  be  several  kinds. 


12  GENERAL  CONCEPTIONS. 

The  uneven  distribution  of  the  yolk  granules  in  the  ova  of  mammals  (compare 
page  34)  indicates  that  there  are  unlike  regions,  the  morphological  significance 
of  which,  however,  is  not  known  yet. 

Two  views  as  to  the  constitution  of  the  ovum  in  relation  to  the  structures 
which  arise  from  it  have  been  brought  forward.  According  to  one  view,  each 
part  of  the  ovum  is  predestined  to  form  a  definite  part  of  the  adult  and  cannot 
form  any  other  part.  According  to  the  other  view,  the  ovum  is  homogeneous 
in  its  essential  lack  of  true  differentiation,  and  any  part  of  it  may  form  any  part 
of  the  adult  if  given  the  requisite  opportunity.  The  first  view  is  known  as  the 
mosaic  Jheory,  the  egg  being  compared  to  a  mosaic,  and  was  founded  by  Wilhelm 
Roux  (1888).  The  second  view  is  known  as  the  theory  of  isotropism  and  was 
founded  by  Ernst  Pfliiger  in  1884. 

Pfliiger's  theory  of  isotropism  was  based  upon-  his  experiments  on  frogs'  eggs. 
Each  egg  has  a  small  white  area  which  normally,  lies  underneath,  the  larger, 
darkly  pigmented  area  of  the  egg  alone  showing  from  above.  Out  of  the  dark 
area  the  back,  with  the  nervous  system  and  other  parts,  takes  its  origin.  If  the 
eggs,  freshly  fertilized,  are  fastened  with  the  white  side  up,  then  the  white  side  pro- 
duces an  absolutely  normal  back  and  nervous  system,  normal  as  to  form  and  func- 
tion, though  lacking  the  typical  pigmentation.  These  observations  were  confirmed 
by  Born,  who  further  discovered  that  the  segmentation  nucleus  always  rises 
toward  the  upper  side  of  the  egg,  and  that  the  position  of  the  nucleus  determines 
which  part  of  the  ovum  shall  become  the  dorsal  side  of  the  embryo.  Another  set  of 
experiments  by  Oskar  Schultze  demonstrated  that  both  the  unpigmented  and  the 
pigmented  sides  of  the  same  egg  could  be  made  to  produce  dorsal  structures.  An- 
other class  of  experiments,  which  were  first  made  by  Hans  Driesch,  has  revealed 
that  the  earliest  cells  (segmentation  spheres,  blastomeres,  or  cleavage  cells,  as  they 
are  variously  called)  produced  by  the  ovum  preserve  the  undifferentiated  qualities 
of  the  parent  egg,  and  may  develop  in  one  way  or  another  according  to  circumstances. 
The  egg  of  a  sea-urchin  divides  into  two  cells,  each  of  which  multiplies  and  nor- 
mally gives  rise  to  half  of  the  body  of  the  animal.  By  somewhat  violent  shaking  the 
two  cells  may  be  artificially  separated;  each  cell  may  then  develop  into  a  complete 
larval  sea-urchin,  but  of  half  the  normal  size  only.  Similar  experiments  have  since 
been  made  by  several  investigators,  who  have  obtained  like  results  with  other 
animals,  vertebrate  as  well  as  invertebrate.  Even  more  remarkable  larvae  have 
been  raised  from  blastomeres  of  the  four-cell  and  eight-cell  stages  of  segmentation, 
producing  larvae  of  one-fourth  and  one-eighth  the  normal  size.  Zoja  claims  to  have 
repeated  the  experiment  successfully  on  the  eggs  of  Clytia  and  to  have  obtained 
one-sixteenth  larvae. 

Roux's  mosaic  theory  was  based  on  W.  His's  principle  of  the  organ-forming 
areas  of  the  germ,  or,  as  it  has  been  also  termed,  the  law  of  germinal  localization. 
His  pointed  out  that  in  the  normal  course  of  development  every  spot  in  the 
blastoderm  corresponds  to  some  future  organ,  and  suggested  that  logically  it  is 


C  Y  TOMORPHOSIS.  1 3 

probable  that  every  organ  is  represented  by  some  region  in  the  ovum  itself.  In 
other  words,  although  the  organs  of  course  do  not  exist  preformed  in  the  egg, 
yet  the  material  for  them  is  there  and  prelocalized.  Roux  has  developed  this 
conception  and  has  compared  the  egg  to  a  mosaic;  each  member  of  the  mosaic 
is  assumed  to  undergo  self-differentiation  or  its  predestined  development.  Com- 
plete verification  of  this  theory  has  not  been  secured,  but  it  has  been  demonstrated 
that  certain  eggs  of  animals  of  several  invertebrate  orders  contain  substances, 
which  have  an  exact  distribution,  and  which  have  a  definite  fixed  relation  to  adult 
structures.  By  putting  some  of  these  eggs  in  a  centrifuge  these  substances  may 
have  their  distribution  artificially  changed.  In  eggs  thus  altered  the  substances 
continue  to  transform  themselves  into  their  predestined  structures,  which  conse- 
quently appear  displaced.  By  these  experiments  the  mosaic  theory  has  received 
a  limited  confirmation. 

At  present  it  is  impossible  to  reconcile  the  two  theories  of  the  constitution 
of  the  ovum,  but  since  both  are  based  apparently  on  facts  it  seems  probable  that 
they  will,  by  wider  knowledge,  be  fused  into  a  single  coherent  conception. 

Meanwhile  the  fact  of  most  importance  in  cytomorphosis  is,  that  the  protoplasm 
of  the  ovum  is  undifferentiated  and  lacks  completely  any  variations  of  its  stnu 
comparable  to  those  which  we  observe  as  characteristic  in  the  cells  of  adult  tis- 
The  potentialities  of  the  ovum,  on  the  other  hand,  are  of  course  very  great.     Experi- 
mental  embryology  is   now  endeavoring  to   ascertain  what  physiological  causes  ren- 
der   those    potentialities    active.      From    physiological    embryology    much    is    to    be 
expected. 

2.  Differentiation. — This  may  be  defined  as  a  process  by  which  the  structure  of 
the  cells  is  modified,  so  that  they  become  dissimilar  by  acquiring  an  organization 
which  adapts  them  to  special  functions.  The  cells  which  arise  during  the  segmen- 
tation of  the  ovum  differ  but  slightly  from  one  another.  As  development  progresses 
we  find  the  cells  change,  some  in  one  way,  some  in  another,  so  that  many  kinds  of 
cells  are  produced,  but  of  each  kind  we  find  a  large  number  of  cells.  Each  kind 
of  cell  may  be  said,  roughly  speaking,  to  form  a  tissue  for  itself.  Cells  of  each 
tissue  offer  visible  peculiarities  by  which  they  may  be  readily  distinguished  from  one 
another  under  the  microscope.  It  thus  appears  that  the  production  of  tissues  is  the 
main  result  of  differentiation,  so  that  this  process  of  development  may  be  fairly  accu- 
rately defined  as  equivalent  to  histogenesis.  As  to  the  factors  which  cause  differen- 
tiation, we  have  no  satisfactory  knowledge.  We  can,  at  present,  only  note  the  changes 
when  they  acquire  such  magnitude  as  to  become  microscopically  visible.  As  to  the 
physiological  conditions  which  cause  these  changes  we  have  almost  no  conceptions. 
It  is  probable  that  the  nucleus  has  a  leading  role  to  play,  but  our  knowledge  of 
this  role  is  too  little  advanced  to  permit  a  profitable  discussion  of  the  subject  here. 

The  actual  process  of  differentiation  shows  itself  both  in  the  protoplasm  and  in 
the  nucleus  of  the  cell.  The  changes  in  the  former  are  the  more  conspicuous,  and 
therefore  the  better  known.  The  changes  in  the  nucleus  have  still  to  be  adequately 


14  GENERAL  CONCEPTIONS. 

studied.  The  changes  in  the  protoplasm  are  twofold:  First,  in  the  intimate  struc- 
ture of  the  protoplasm  itself  and  in  the  size  and  disposition  of  its  strands  and  fila- 
ments; secondly,  in  the  character  of  the  various  substances  to  be  found  imbedded 
in  the  protoplasm.  These  two  kinds  of  change  are  well  illustrated,  the  first,  by 
the  nerve-cells;  the  second,  by  the  gland-cells,  for  instance,  in  the  pancreas.  The 
student  can  easily  see  that  the  character  of  the  protoplasm  in  the  adult  nerve-cell 
differs  profoundly  from  that  of  a  cell  from  one  of  the  embryonic  germ-layers,  hav- 
ing become  visibly  much  more  complex.  In  the  secretory  cells  of  the  pancreas  the 
zymogen  granules  are  conspicuous;  their  distribution,  uniform  size,  and  refractile 
qualities  demonstrate,  immediately  their  unlikeness  to  anything  found  in  the  embry- 
onic cells.  These  granules  are  not  protoplasm,  but  particles  imbedded  in  the  proto- 
plasm or,  as  they  may  be  called,  enclosures. 

The  Law  of  Genetic  Restriction. — Another  fundamental  idea,  which  it  is  most 
important  for  the  student  to  grasp,  is  that  differentiation  acts  as  a  progressive 
restriction  upon  the  further  development.  Each  successive  stage  of  differentiation 
puts  a  narrower  limitation  upon  the  possibility  of  further  advance. 

The  range  of  possible  changes  at  any  given  time  is  determined  not  merely  by 
the  nature  or  kind,  but  also  by  the  stage  or  degree  of  the  previous  differentiation. 
The  law  of  genetic  restriction  governs  the  entire  ontogeny.  In  order  to  illustrate 
it  and  to  emphasize  it,  it  will  be  profitable  to  consider  a  few  illustrations  from  each 
of  the  germ-layers.  First,  then,  the  ectoderm.  This  layer  early  separates  into  two 
parts,  one  to  form  the  nervous  system,  the  second  the  epidermis;  the  nervous  part 
thereafter  never  forms  epidermal  structures,  the  epidermal  part  never  forms  a  medul- 
lary canal.  The  central  nervous  system  retains  in  part  a  simple  epithelial  charac- 
ter (ependyma  proper),  but  most  of  its  walls  become  nervous  tissue;  its  cells  pass 
from  the  indifferent  stage  and  become  neuroglia  cells  or  young  nerve-cells  (neuro- 
blasts).  Neuroglia  cells  never  become  anything  else,  and  the  nerve-cells  are  always 
nerve-cells  to  the  end.  Next,  as  to  the  entoderm.  Wherever  in  it  specialization 
takes  place,  as  in  the  tonsil,  thymus,  thyroid,  oesophagus,  liver,  or  pancreas,  each 
territory  of  cells  keeps  its  characteristics  and  never  assumes  those  of  another  terri- 
tory. Finally,  as  to  the  mesoderm.  It  is  found  very  early  to  include  in  vertebrate 
embryos  four  kinds  of  cells,  of  which  the  most  numerous  are  the  undifferentiated 
cells,  the  other  three  kinds  being  the  endothelium  -of  blood-vessels,  red  blood-cells, 
and  germ-cells.  All  of  these  are  precociously  specialized;  they  are  few  in  number, 
yet  they  are  probably  the  parents  of  all  the  cells  which  are  produced  of  their  kind 
each  throughout  life.  Passing  on  to  a  later  stage,  we  note  that  when  a  striated 
muscle-fiber  is  produced  a  striated  muscle-fiber  it  always  remains,  and  it  never 
becomes  anything  else. 

Two  Types  of  Differentiation.— There  are  two  distinct  types  of  cell  differentia- 
tion which  I  think  have  not  hitherto  been  clearly  recognized  or  defined.  For  both 
types  the  starting-point  is  the  same— the  undifferentiated  embryonic  cell.  In  one 
type  we  find  that,  as  the  cells  proliferate,  a  portion  of  them  only  undergoes  differ- 


CYTOMORPHOSIS.  15 

entiation,  and  another  portion  remains  more  or  less  undifferentiated  and  retains 
more  or  less  fully  the  power  of  continued  proliferation.  The  epidermis  is  a  good 
representative  of  this  type.  Its  basal  layer  consists  of  embryonic  cells,  which 
multiply;  some  of  these  cells  move  into  the  upper  layers,  enlarge,  and  differentiate 
themselves  into  horny  cells;  others  remain  in  the  basal  layer  and  continue  to  mul- 
tiply. The  progeny  of  a  given  basal  epidermal  cell  do  not  all  have  the  same  fate, 
but  divide  themselves  into  two  kinds  of  cells,  one  kind  retaining  the  ancestral 
character,  the  other  becoming  something  new  and  unlike  the  parent  cell.  Differ- 
entiation according  to  the  second  type  is  characterized  by  its  inclusion  of  all  the 
cells.  This  type  has  its  culminating  and  most  perfect  illustration  in  the  central 
nervous  system,  where  comparatively  early  in  embryonic  life  all  the  cells  become 
specialized,  and  with  the  acquisition  of  specialization  they  forfeit  their  power  of 
multiplication— the  neuroglia  cells  partly,  the  nerve-cells  wholly.*  The  growth  of 
the  brain  after  early  stages  depends  not  on  the  proliferation  of  cells,  but  chiefly 
upon  the  increase  in  size  of  the  individual  cell.  The  correctness  of  this  statement 
is  not  affected,  in  my  belief,  by  the  fact  that  epithelial  portions  of  the  medullary 
tube  in  comparatively  late  stages  may  be  added  to  the  nervous  portion,  the  cells 
multiplying  rapidly,  as  we  see  at  the  growing  edge  of  the  young  cerebellum.  The 
brain  here  grows  by  the  addition  of  cells  in  the  indifferent  stage,  but  as  soon  as 
these  cells  are  differentiated  they  conform  to  the  general  law  and  divide  no  more 
(neurones)  or  slowly  (glia  cells). 

The  importance  to  pathologists  of  a  thorough  knowledge  of  the  genesis  of  the 
tissues  from  their  germ-layers  can  hardly  be  emphasized  too  strongly,  for  it  is  more 
than  probable  that  all  pathological  tissues  are  as  strictly  governed  by  the  law  of 
genetic  restriction  as  are  the  normal  tissues. 

3.  Regression. — The  use  of  this  term  does  not  imply  that  a  cell  can  move 
backward  after  differentiation  into  a  stage  of  lower  differentiation  or  into  an  un- 
differentiated condition.  So  far  as  we  know  at  present,  such  a  change  does  not 
occur,  and  we  therefore  look  upon  it  as  impossible.  Regressive  changes  are  very 
unlike  the  constructive  changes  which  appear  in  differentiation,  for  they  are  destruct- 
ive. They  fall  into  three  main  groups:  first,  changes  of  direct  cell  death;  second, 
necrobiosis  or  indirect  cell  death  preceded  by  changes  in  cell  structure;  third,  hyper- 
trophic  degeneration  or  indirect  cell  death  preceded  by  growth  and  structural 
change  of  the  cell,  often  with  nuclear  proliferation.  Direct  cell  death  implies  that 
the  cell  loses  its  vitality,  and,  being  dead,  disintegrates;  or,  may  be,  is  removed  by 
some  means,  chemical  or  phagocytic,  before  disintegration  occurs.  Necrobiosis  and 
hypertrophic  degeneration  are  normal  processes,  which  invariably  occur  in  the 
normal  body  and  play  an  important  role  in  its  development.  Without  their  occur- 
rence on  a  large  scale  the  normal  round  of  human  life  would  be  impossible.  The 
student  should  •  free  himself  from  the  unfortunate  tradition  that  these  processes  are 
exclusively  pathological. 

*  With  possibly  very  rare  exceptions. 


16  GENERAL  CONCEPTIONS, 

Correct  notions  on  this  subject  are  so  important  that  a  few  illustrations  may 
be  mentioned.  Let  us  begin  with  necrobiosis.  There  are  organs  whose  existence 
is  limited  in  time,  such  as  the  thymus  and  foetal  kidney.  These  organs  attain 
their  full  differentiation,  and  thereafter  most  of  their  elements  die  off  and  finally 
are  resorbed,  most  of  the  organ  disappearing.  Another  familiar  illustration  is 
offered  by  hairs,  which  die  and  are  shed.  Cell  death  on  a  large  scale  is  a  com- 
mon phenomenon  of  the  tissues.  It  occurs  in  the  cartilage  both  when  the  cartilage 
is  permanent  and,  even  more  conspicuously,  when  the  cartilage  gives  way  to  bone, 
the  disintegration  of  the  cartilage  cells  preceding  the  irruption  of  the  bone-forming 
tissues.  It  occurs  among  the  gland-cells  of  the  intestine,  in  the  pregnant  uterus, 
and  in  all  the  tissues  of  human  decidua  reflexa.  Degeneration  in  the  stricter 
sense  of  the  ante-mortem  and  hypertrophic  change  of  cell  structure  is  also  of  wide- 
spread occurrence  in  the  healthy  body.  Perhaps  no  instance  of  this  is  more 
familiar  than  the  production  of  horny  tissue  in  the  epidermis  or  elsewhere.  That 
fatty  degeneration  takes  place  normally  has  long  been  taught,  while  mucoid  and 
colloid  degeneration  are  so  obviously  normal  that  we  commonly  think  of  their 
pathological  occurrence  as  merely  an  exaggeration  of  a  normal  state.  Hypertrophic 
degeneration  is  an  extremely  common  pathological  process,  but  it  also  occurs  as  a 
normal  process,  as,  for  example,  in  epidermal  cornification,  as  just  mentioned,  and 
very  strikingly  in  the  production  of  giant-cells  (myeloplaxes,  etc.),  and  on  an  astound- 
ing scale  in  the  uterine  tissues  during  pregnancy  in  many,  perhaps  all,  mammals. 

4.  The  Removal  of  Cells. — The  sloughing  off  of  cells  is  one  of  the  most  familiar 
phenomena,  since  it  occurs  incessantly  over  the  epidermis  and  with  hairs.  Its  part 
in  menstruation  and  its  colossal  role  in  the  after-birth  are  known  to  all,  and  every 
practitioner  is  accustomed  to  look  for  shed  cells  in  urinary  sediment.  Large 
numbers  of  cells  are  lost  by  the  intestinal  epithelium.  The  destruction  of  blood- 
corpuscles  is  incessant,  and  we  might  greatly  extend  the  list  of  these  illustrations. 
Owing  to  the  enormous  loss  of  cells  to  which  the  body  is  subject,  there  is  provision 
to  make  good  this  loss.-  This  provision  is  called  "regeneration,"  and  has  been 
dealt  with  in  an  enormous  number  of  investigations.  During  embryonic  life 
regeneration  plays  a  comparatively  insignificant  part,  and  we  shall  not  have  to  deal 
with  it  further. 

Of  the  four  stages  of  cytomorphosis,  the  second,  or  stage  of  differentiation,  is 
that  which  will  principally  claim  our  attention.  But  we  cannot  fully  understand 
the  developmental  processes  unless  we  also  have  constantly  in  mind  the  normal 
degeneration  and  death  of  cells,  even  in  the  embryo. 

Comparison  of  Larval  and  Embryonic  Types  of  Development. 

We  have  seen  in  the  preceding  section  that  the  first  cells  produced  in  develop- 
ment from  the  ovum  are  undifferentiated,  and  are  capable  of  development  in  many 
and  varied  directions.  The  more  they  become  specialized,  the  more  their  possi- 
bilities of  further  varied  development  are  decreased.  It  is  thus  obvious  that  the 


COMPARISON  OF  LARVAL  AND  EMBRYONIC  TYPES.  17 

greater  the  number  of  cells  of  the  undifferentiated  type  that  can  be  produced,  the 
greater  will  be  the  number  of  elements  which  can  be  later  differentiated.  Hence, 
the  more  the  period  for  the  production  of  undifferentiated  cells  is  prolonged  and 
the  commencement  of  differentiation  postponed,  the  more  complex  may  be  the 
degree  of  organization  ultimately  attainable. 

It  is  convenient  to  designate  the  undifferentiated  cells  as  they  arise  from  the 
segmentation  of  the  ovum  by  the  term  "embryonic  cells.1'  The  object  of  this 
section  is  to  point  out  that  the  larval  type  of  development  is  less  favorable  for  the 
multiplication  of  embryonic  cells  than  is  the  embryonic  type;  and,  further,  that  the 
embryonic  type  becomes  more  and  more  marked  as  we  ascend  in  the  animal 
kingdom. 

The  Larval  Type. — In  the  lower  multicellular  animals  we  encounter  only  larvae; 
sponges,  jellyfish,  starfish,  and  worms  all  pass  through  their  early  stages  as  larvae. 
Now,  larvae  are  animal  forms  which  have  to  obtain  their  own  food  and  to  protect 
themselves  against  enemies.  They  are,  therefore,  provided  with  a  variety  of  organs, 
or,  as  we  may  say,  with  differentiated  tissues  which  enable  them  to  perform  the 
various  physiological  functions  which  "are  necessary  for  the  maintenance  of  their 
existence.  The  differentiation  of  tissues  comes  in  very  early. 

The  Embryonic  Type. — True  embryos  arise  from  eggs  which  contain  a  more  or 
less  considerable  amount  of  yolk  or  nutritive  material,  the  presence  of  which  renders 
unnecessary  any  activity  on  the  part  of  the  embryo  to  obtain  its  food-supply;  and 
we  find,  moreover,  that  these  embryos  are  protected  by  hard  shells  or  other  devices 
from  their  enemies.  Their  only  task  is  to  pursue  their  own  development.  Under 
these  circumstances  it  is  possible  for  the  embryos  to  continue  for  a  long  time  the 
production  of  embryonic  cells,  and  we  observe  that  the  beginning  of  the  differen- 
tiation proper  is  correspondingly  postponed.  The  transition  from  the  larval  to  the 
embryonic  type  is  very  gradual.  The  yolk  appears  in  the  lower  animals  in  small 
quantities,  increasing  in  some  of  the  higher  types,  and  attaining  its  maximum  in 
some  of  the  highest.  Since  the  embryo  is  dependent  on  the  yolk,  and  since  the 
yolk  exists  only  in  the  higher  forms  in  sufficient  quantities,  'it  follows  that  fully 
typical  embryos  can  occur  exclusively  in  the  higher  animal  types. 

In  the  mammalia  the  ovum  contains  a  rather  small  quantity  of  yolk,  yet  the 
mammals  are  the  highest  afiimals  and  develop  most  perfectly  according  to  the  em- 
bryonic type.  This  peculiarity  is  due  to  the  fact  that  two  special  physiological 
devices  have  been  evolved  in  the  mammals  to  supply  food  to  the  developing  em- 
bryo. First,  there  is  a  special  relation  established  between  the  embryo  and  the 
uterus  by  means  of  a  complicated  adjustment  of  embryonic  and  uterine  tissues, 
which  supplies  nutrition  to  the  embryo  from  the  blood  of  the  mother.  Secondly, 
there  are  the  mammary  glands,  which  also  serve  the  same  function.  By  these  two 
devices  the  embryo  is  even  more  completely  freed  from '  the  necessity  of  seeking  its 
food  and  protecting  itself  than  is  the  case  with  those  forms,  such  as  the  birds  or 
elasmobranchs,  in  which  the  supply  of  food  material  is  very  large. 


18  GENERAL  CONCEPTIONS. 

Germ-layers. 

The  germ-layers  are  the  first  groups  of  cells  to  arise  as  the  result  of  the  seg- 
mentation of  the  ovum.  They  are  three  in  number,  and  each  forms  a  distinct  sheet 
or  lamina.  As  stated  on  page  10,  these  three  primitive  layers  are  termed  "ecto- 
derm," "mesoderm,"  and  "entoderm. "  The  ectoderm  is  the  most  external  of  the 
three,  and  upon  the  outside  of  the  body  parts  of  the  ectoderm  remain  permanently  to 
constitute  the  outside  skin  or  epidermis.  From  its  very  position  it  necessarily  is  the 
part  of  the  body  to  come  into  relation  with  the  external  world,  and  accordingly  we  find 
that  its  two  great  duties  are  to  produce  the  protective  covering  of  the  body  and  the 
apparatus  for  receiving  and  utilizing  sensations;  in  other  words,  the  chief  sensory  or- 
gans and  the  nervous  system.  The  entoderm,  on  the  contrary,  forms  the  internal 
cavity  of  the  digestive  canal  and  its  appendages.  It  therefore  is  concerned  chiefly 
with  the  production  of  the  organs  of  digestion,  and  appears  in  the  adult  as  the 
epithelium  of  the  digestive  and  respiratory  organs  and  of  the  glands  appended  to 
the  digestive  tract.  The  mesoderm,  lying  as  it  does  between  the  other  two  layers, 
is  shut  off  by  them  from  direct  relation  with  the  external  world  or  with  food- 
matter,  and  is  accordingly  restricted  to  a  series  of  internal  functions,  of  which  four 
are  especially  important:  (i)  The  function  of  circulation  both  of  blood  and  lymph 
through  definite  channels;  (2)  of  excretion;  (3)  of  movement;  (4)  of  supporting  the 
body,  especially  the  parts  produced  from  the  ectoderm  and  entoderm.  It  is  from 
the  middle  germ-layer,  therefore,  that  the  connective  and  skeletal  tissues  arise,  that 
the  muscular  tissues  arise,  that  the  excretory  organs  arise,  and  that  the  blood, 
blood-vessels,  and  lymphatics  arise. 

The  inner  and  outer  germ-layers  are  primarily  simple  epithelial  structures,  con- 
sisting each  of  a  single  layer  of  cells.  This  primitive  characteristic  is  never  wholly 
obliterated  and  really  controls  all  of  the  modifications  which  these  two  layers 
undergo.  The  mesoderm,  on  the  other  hand,  is  primarily  not  epithelial,  but  mesen- 
chymal.  Mesenchyma  consists  of  widely  separated  cells  which  form  a  continuous  network  of 
protoplasm,  the  meshes  of  which  are  originally  filled  by  a  homogeneous  intercellular 
substance  or  matrix.  The  student  will  have  frequent  occasion  in  his  practical  work 
to  study  it  in  its  embryonic  stages. 

The  Caelom. — The  ccelom  is  the  primitive  body-cavity  of  the  embryo.  It  arises 
as  a  space  in  the  mesoderm.  Soon  after  this  space  has  appeared  we  find  that  the 
cells  of  the  mesoderm,  which  bound  it,  assume  an  epithelial  character;  consequently 
the  mesoderm,  after  the  coelom  has  appeared,  consists  of  mesenchyma  and  of  an 
epithelial  layer  bounding  the  coelom.  This  epithelial  layer  is  called  the  mesothelium. 
The  mesoderm,  therefore,  differs  fundamentally  from  the  ectoderm  and  entoderm 
by  this  peculiarity,  that  it  comprises  both  an  epithelial  and  a  non-epithelial  portion. 
Both  portions  play  very  important  roles  in  the  production  of  the  various  tissues  and 
organs  of  the  body.  There  is  another  respect  also  in  which  the  mesoderm  differs 
from  the  other  germ-layers,  for  we  find  that  it  increases  in  volume  and  in  com- 
plexity as  we  asce»d  from  the  lower  to  the  higher  types  of  animals,  or  as  we. pass 


GERM-LA  YERS. 


19 


from   the   embryo   toward  the   adult  condition,   more   than   does   either  the   outer  or 
inner  germ-layer. 

The  Specific  Quality  of  the  Germ-layers.— Each  germ-layer  has  its  specific  and 
exclusive  function  in  the  production  of  tissues,  giving  rise  only  to  the  tissues  which 
are  proper  to  it  and  never  to  the  tissues  which  are  proper  to  either  of  the  other 
layers.  We  must,  indeed,  so  far  as  -  our  present  knowledge  goes,  regard  most,  at 
least,  of  the  cells  in  the  germ-layers  as  originally  wholly  indifferent  as  individual 
cells.  But  we  must,  nevertheless,  not  forget  that,  as  members  of  a  germ-layer, 
their  potential  fate  is  already  restricted.  It  is  probable,  if  we  could  successfully 
transplant  an  undifferentiated  cell  from  one  germ-layer  to  another,  that  it  could 
take  part  in  the  production  of  the  tissues  proper  to  that  layer.  But  it  is  further 
probable  that  this  would  be  impossible  after  the  differentiation  of  the  cells  in  any 
layer  had  fairly  begun.  After  a  cell  has  become  definitively  a  member  of  one  of 
the  germ-layers,  it  probably  never  migrates  to  join  another  layer.  The  accompany- 
ing table  presents  the  principal  tissues  classified  according  to  the  layers  to  which 
they  belong.  There  have  been  classifications  of  organs  on  the  germ-layer  basis 
published  before,  but  inasmuch  as  organs  usually  contain  cells  from  two  layers,  we 
get  a  more  correct  presentation  of  the  actual  genetic  relationship  by  confining  our 
tabulation  to  the  tissues.  Blood-vessels  and  blood-cells  arise  very  early,  before  the 
clear  separation  of  the  mesoderm  and  entoderm  has  occurred.  It  is  possible  that  they 
are  entodermal.  With  these  two  limitations  the  table  presents  our  present  knowledge. 


(A)    ECTODERMAL. 

Epidermis. 

a.  epidermal  appendages, 

b.  lens  of  eye. 
Epithelium  of 

a.  cornea, 

b.  olfactory  chamber, 

c.  auditory  organ, 

d.  mouth 

(oral  glands), 
(enamel  organ), 
(hypophysis), 

e.  anus, 

/.  chorion, 

fetal  placenta, 
g.  amnion. 
Nervous  system. 

a.  brain, 

optic  nerve, 
retina, 

b.  spinal  cord, 

c.  ganglia, 

d.  neuraxons, 

e.  chromaffme  cells. 


CLASSIFICATION  OF  THE  TISSUES. 

(B)  MESODERMAL. 

1.  Mesothelium. 

a.  epithelium  of 

peritoneum, 
pericardium, 
pleura, 
urogenital  organs, 

b.  striated  muscles. 

2.  Mesenchyma. 

a.  blood-cells  (red  and  white), 

b.  blood-vessels, 

c.  connective  tissue, 

cellular  reticulum, 
smooth  muscle, 
pseudo-endothelium, 
fat  cells, 
pigment  cells, 

d.  lymphatics, 

e.  spleen, 

/.  supporting  tissues, 

cartilage, 

bone, 
g.  marrow  of  bone. 


(C)  ENTODERMAL. 

1.  Notochord. 

2.  Epithelium  of 

a.  digestive  tract, 

oesophagus, 

stomach, 

liver, 

pancreas, 

small  intestine, 

yolk-sac, 

large  intestine, 

caecum, 

vermix, 

rectum, 

allantois  (bladder), 

b.  pharynx, 

Eustachian  tube, 

tonsils, 

thymus, 

parathyroids,' 

thyroid, 

c.  respiratory  tract, 

larynx, 

trachea, 

lungs. 


20  GENERAL  CONCEPTIONS. 

The  Constitution  of  Organs. — Few  organs  are  formed  from  a  single  germ-layer, 
for  as  we  find  organs  in  the  vertebrate  body  they  usually  consist  of  two  parts,  one 
of  which  may  be  regarded  as  the  part  proper  of  the  organ,  upon  which  the  per- 
formance of  its  special  function  directly  depends,  and  the  accessory  part,  which 
supplies  the  necessary  physiological  conditions  for  the  functioning  of  the  organ. 
For  example:  in  a  salivary  gland  the  actual  work  of  secretion  is  performed  by  the 
epithelial  cells  of  the  gland,  but  these  cells  cannot  act  unless  they  are  supported 
by  connective  tissue  and  supplied  with  blood  and  lymph,  three  conditions  which 
depend  upon  the  mesoderm,  and  also  supplied  with  nerves,  a  condition  which 
depends  upon  the  ectoderm.  By  far  the  majority  of  organs  have  their  functional 
part  produced  from  epithelium,  and  this  epithelium  may  come  either  from  the 
original  outer  or  inner  germ-layer,  as  the  case  may  be,  or  from  the  mesothelial 
portion  of  the  middle  layer.  But  the  organ,  as  a  whole,  requires  for  its  comple- 
tion the  addition  of  other  elements,  as  indicated  in  the  example  given.  We  find, 
therefore,  that  there  are  no  adult  organs  which  are  constituted  solely  by  either  the 
ectoderm  or  entoderm,  although  there  are  organs  the  principal  part  of  which  may 
come  from  one  or  the  other  of  these  germ-layers,  but  to  complete  the  organ  the 
mesoderm  must  help.  On  the  other  hand,  the  mesoderm  may  form  complete 
organs  by  itself,  or  at  least  with  no  other  aid  from  the  other  germ-layers  than  is 
given  by  the  supplying  of  nerve-fibers.  Such  purely  mesodermal  organs  are  illus- 
trated by  the  spleen,  the  kidney,  and  the  sexual  glands. 


The  Relations  of  Surface  to  Mass. 

However  much  the  weight  of  an  animal  increases  during  its  development,  the 
ratio  of  the  free  surface  to  the  mass  alters  but  slightly  from  the  ratio  established 
when  the  embryo  begins  to  take  food  from  outside.  It  is  only  for  convenience 
that  I  express  this  law  in  this  precise  form;  in  reality,  about  it  our  knowledge  is 
scanty  and  our  conceptions  vague.  According  to  a  geometrical  principle,  when  the 
bulk  of  a  body  bounded  by  a  simple  surface  increases,  the  surface  enlarges  less 
than  the  mass— in  the  simplest  case  of  a  cube,  the  surface  increases  as  the  square, 
the  mass  as  the  cube,  of  the  diameter.  If  in  a  cube  of  unit  diameter  one  unit 
of  surface  bounds  one  unit  of  mass,  then  in  a  cube  of  three  units  diameter  nine 
units  of  surface  will  bound  twenty-seven  units  of  mass;  the  proportion  in  the  first 
cube  is  1:1,  in  the  second  1:3.  To  maintain  the  proper  proportion  in  the  embryo, 
simple  enlargement  is  insufficient,  therefore  the  surface  increases  by  becoming  more 
and  more  irregular.  The  irregularities  are  characteristic  of  each  organ  and  part, 
and  may  be  either  large  or  microscopic.  They  may  be  conveniently  grouped  under 
two  main  heads — projections  and  invaginations. 

Projections  are  illustrated  by  the  limbs,  filaments  of  the  gills  in  fishes,  the 
villi  of  the  intestine,  folds  of  the  stomach  in  ruminants,  etc.  In  every  case  the 
projection  is  covered  by  an  epithelium  and  has  a  core  of  mesodermic  tissue. 


THE  RELATIONS  OF  SURFACE  TO  MASS.  21 

furaginations  exist  in  much  more  varied  form  and  play  a  principal  part  in 
the  differentiation  of  the  animal  body.  They  may  be  classified  under  four  principal 
heads:  (i)  dilatations;  (2)  diverticula;  (3)  glands;  (4)  vesicles.  Dilatations  have 
considerable  importance  in  embryology:  the  stomach,  lungs,  bladder,  and  uterus 
arise  as  gradual  dilatations  of  canals  or  tubes  of  originally  nearly  uniform  diam- 
eters. Diverticula^  in  the  sense  of  relatively  large  blind  pouches,  also  form  impor- 
tant organs,  such  as  the  caecum  and  appendix  vermiformis,  or  the  gall-bladder; 
these  structures  arise  each  as  a  blind  outgrowth  of  a  canal,  the  walls  of  which  at 
a  certain  point  rapidly  grow  to  form  the  pouch.  Glands  are,  as  first  shown  by 
Johannes  Miiller's  classic  researches,  only  small  diverticula,  which  end  blindly  and 
appear  in  an  immense  variety  of  modifications;  the  manifold  types  of  glands  are 
discussed  below  in  a  separate  paragraph;  they  constitute  the  largest  class  of  organs 
with  which  we  have  to  deal.  The  glands  are  developed  from  epithelium  and  push 
their  way  into  the  mesoderm  upon  which  the  epithelium  rests,  while  in  dilatations, 
and  in  diverticula,  .the  epithelium  and  mesoderm  expand  together.  Vesicles  we  call 
those  epithelial  sacs  which  develop  somewhat  like  glands  by  growing  into  the  meso- 
derm, but  the  mouth  of  the  invagination  closes  by  the  coalescence  of  the  epithelium, 
thus  shutting  the  cavity.  The  closed  sac  separates  from  the  epithelium  from  which 
it  arose,  and  connective  tissue  grows  between  the  two;  the  sac  may  then  undergo 
various  modifications.  The  membranous  labyrinth  of  the  ear  is  developed  from  the 
ectoderm  in  this  way,  as  is  also  the  lens  of  the  eye.  We  might  perhaps  also  class 
the  medullary  canal  under  this  head  (cf .  Chapter  V)  if  we  choose  to  consider  it  as 
a  vesicle  so  much  lengthened  that  it  has  become  a  tube. 

Glands. — A  gland  may  be  defined  as  a  structure  which  produces  material  which 
is  discharged  from  the  gland  and  used  elsewhere  to  meet  a  physiological  need.  Ac- 
cording to  the  nature  of  this  material,  we  distinguish  two  fundamentally  different  types 
of  so-called  glands.  One  of  these  we  designate  as  the  true  glands,  which  produce 
chemical  substances  which  are  thrown  off  from  the  cells  producing  them  to  constitute 
the  secretion  of  the  gland,  so  that  the  cells  themselves  all  remain  in  the  gland.  In 
the  second  type  the  cells  themselves  are  multiplied,  so  that  the  structure  yields,  as 
it  were,  a  crop  of  cells,  which  is  removed  from  the  site  of  origin  and  then  utilized 
physiologically.  Glands  of  this  type  may  be  called  cytogenic.  Of  the  true  glands 
we  may  distinguish  several  sorts.  The  simplest  kind  consists  of  a  single  cell.  Of 
unicellular  glands,  the  goblet  cells  are  the  most  familiar  type  known  in  man.  Most 
true  glands,  however,  comprise  many  cells  and  are  -classed  as  multicellular.  The 
majority  of  these  have  an  internal  cavity,  which  may  be  simple  or  very  complicated 
in  its  form,  but  is  always  bounded  by  a  layer  of  epithelium.  They  have  in  addi- 
tion a  canal,  which  leads  from  the  cavity  of  the  gland  to  an  external  opening, 
and  is  called  the  duct.  When  the  secretion  is  produced,  the  chemical  substances 
formed  by  the  epithelial  cells  are  discharged  into  the  cavity  of  the  gland  and  thence 
flow  through  the  duct  to  the  outlet.  In  certain  cases  a  remarkable  modification 
may  occur  by  the  obliteration  of  the  duct,  thus  producing  a  so-called  ductless  epi- 


22  GENERAL  CONCEPTIONS. 

thelial  gland.     In  such  structures  the  secretion  can  escape  only  by  transfusion  into 
the  blood  or  lymph. 

The  epithelial  glands  with  ducts  exhibit  two  main  sorts  of  modification,  for 
they  are  either  small  structures  which  occur  in  great  numbers,  or  they  are  larger 
structures,  each  constituting  a  separate  organ.  Hence,  we  divide  the  glands  into 
simple  glands  and  compound  or  organic  glands.  The  simple  glands  are  always 
small  and  have  one  or  several  centers  of  growth,  according  as  they  are  simple 
tubes  or  slightly  branching.  Those  of  each  kind  are  always  very  numerous,  and 
occur  more  or  less  together  over  considerable  areas.  Good  illustrations  of  simple 
glands  are  offered  by  the  sweat  glands  and  the  intestinal  glands.  The  compound 
glands  are  of  greater  bulk.  They  are  provided  with  a  single  main  duct  which  is 
more  or  less  branched,  each  branch  connecting  finally  with  the  secretory  portion 
proper  of  the  organ,  which  portion  may  itself  also  be  branched.  Each  gland  fall- 
ing in  this  division  is  a  more  or  less  complete  organ  by  itself,  receiving  its  special 
blood  supply  and  its  special  innervation — it  is,  in  short,  a  clearly  marked  physio- 
logical entity.  A  specially  striking  morphological  modification  in  the  structure  of 
compound  glands  is  offered  by  the  liver.  In  this  organ,  branches  of  the  glands 
unite  and  form  an  anastomosing  gland  structure,  in  connection  with  which  we 
observe  that  the  branches  of  the  gland  are  not  associated  with  a  development  of 
connective  tissue  and  of  blood  capillaries  between  the  epithelial  elements  of  the 
organ,  as  in  other  compound  glands;  but  are  associated,  on  the  contrary,  with  the 
presence  of  a  sinusoidal  circulation.  We  must  therefore  regard  the  liver  as  a  type 
by  itself. 

Another  class  of  secreting  organs  may  be  termed  false  glands,  as  they  never 
have  ducts  at  any  stage  of  their  development.  Their  chemical  product  is  termed 
an  internal  secretion,  and  is  removed  by  transfusion  into  the  blood.  One  division 
of  the  false  glands  is  derived  from  the  growth  of  epithelium,  while  another  division 
arises  by  modifications  in  mesenchymal  cells.  As  regards  the  glands  of  the  cyto- 
genic  class,  we  have  to  deal  with  those  which  produce  the  free  wandering  cells,  of 
which  the  most  familiar  example  is  the  white  corpuscle  of  the  blood;  those  which 
produce  the  red  corpuscles  of  the  blood;  and,  finally,  those  which  produce  the 
genital  elements. 

As  the  student  proceeds  in  his  study  of  embryology,  he  will  have  clear  illustra- 
tions of  the  development  and  morphology  of  all  the  various  sorts  of  glands.  He 
will  find  it  advantageous,  as  his  acquaintance  with  glands  increases,  to  consult  the 
classification  of  glands  as  presented  in  the  following  table,  based  upon  the  very 
important  morphological  distinctions  pointed  out  in  the  preceding  paragraph. 
Formerly  the  classification  of  glands  was  based  upon  relatively  unimportant  details 
of  their  microscopic  form,  and  not  upon  true  morphological  differences.  Hence 
the  classification  here  proposed  differs  radically  from  those  in  vogue  up  to  the 
present  time. 


CLASSIFICATION  OF  GLANDS.  23 

Classification   of  Glands.* 

Class  A.     Unicellular. 

Class  B.     True  Glands,  always  developed  with  ducts. 

Division  i.    Simple  Glands  (unifollicular  or  single  glands). 

a.  Ectodermal. 

1.  Sweat  glands. 

2.  Sebaceous  glands. 

3.  Buccal  glands. 

b.  Entodermal. 

-    i.  (Esophageal. 

2.  Gastric. 

3.  Intestinal. 

c.  Mesothelial. 

i.  Uterine. 

Division  2.  Compound  Glands  (organic  or  true  compound  glands). 
Type  a,  ductic  epithelial  branching  (with  capillary  circulation). 

1.  Ectodermal. 

Salivaries,  tear  gland,  Harderian. 
Mammary. 

2.  Entodermal. 

Pancreas. 

3.  Mesothelial. 

Appendicular  glands  of  the  urogenital  system. 
Type  b,  ductic  anastomosing  (with  sinusoidal  circulation). 

1.  Liver. 

2.  (Paraphysis  ?) 

Type  c,  ductless  epithelial  (with  secondary  obliteration  of  the  duct). 

1.  Thyroid. 

2.  Hypophysal  gland. 

3.  Infundibular  gland. 

4.  Pineal  gland  (epiphysis). 

Class  C.   False  Glands,  never  developed  with  ducts. 

Division  i.  Epithelioid  Glands  (?  exclusively  entodermal). 

1.  Parathyroid. 

2.  Islands  of  the  pancreas. 

3.  Carotid. 

4.  Thymus. 

Division  2.  Mesenchymal  ductless  Glands. 

1.  Suprarenal  cortex. 

2.  Coccygeal  gland. 

3.  Interstitial  cells  of  genital  glands. 
Class  D.     Cytogenic  Glands. 

Division  i.  Lymphaal  structures;  producing  mesamceboids . 

1.  Lymph  glands  and  follicles. 

2.  Hemolymph  glands. 


*  The  present  table  is  a  modification  of  the  classification  of  glands  proposed  by  the  author  in  1905  (Amer. 
Journ.  Anat.,  iv,  256).  The  principal  change  is  in  putting  the  cytogenic  glands  in  a  class  by  themselves,  as  should 
have  been  done  originally. 


24  GENERAL  CONCEPTIO.\S. 

3.  Spleen. 

4.  (?)  Tonsils  and  thymus. 
Division  2.  Sdnguijactive  organs. 

i.  Bone  marrow. 
Division  3.  Genital  glands. 

1.  Ovary. 

2.  Testis. 

The  Law  of  Unequal  Growth. 

The  changing  shapes  of  the  embryo  and  the  development  of  the  irregularities- 
projections  and  imaginations — which  preserve  the  proper  proportion  between  the 
surface  and  the  mass  of  the  body,  both  depend  upon  the  unequal  growth  of  the 
germ-layers,  especially  in  superficies.  The  expansion  of  a  germ-layer  having  the 
epithelial  type  of  structure*  may  take  place  by  three  means:  (i)  the  multiplication 
of  the  cells;  (2)  the  flattening  out  of  the  cells;  (3)  enlargement  of  the  cells.  In 
the  early  stage's  of  development  the  influence  of  the  first  two  factors  predominates; 
during  the  later  stages,  especially  after  birth,  the  latter.  Of  the  three  factors,  the 
first  is  the  most  important. 

The  unequal  multiplication  of  the  cells  in  all  embryonic  epithelia  is  the  funda- 
mental factor  of  development,  and  we  see  it  shaping  the  embryo,  its  organs,  and 
the  parts  of  organs,  before  histological  differentiation  really  begins.  The  distinct 
areas  and  centers  of  growth  which  are  necessary  to  develop  the  human  body  out 
of  the  germ-layers  are  innumerable,  and  their  distribution,  limitations,  and  inter- 
actions make  up  a  large  part  of  the  subject-matter  of  embryology.  At  every  turn 
of  our  studies  we  encounter  fresh  illustrations.  If  in  a  limited  area  of  a  cellular 
membrane  there  occurs  a  growth  of  expansion  more  rapid  than  in  the  neighboring 
parts,  then  that  area  is,  as  it  were,  bounded  by  a  fixed  ring,  and  can,  therefore, 
find  room  for  its  own  expansion  only  by  rising  above  the  level  of  the  membrane; 
thus,  when  in  the  embryonic  region  of  the  blastodermic  vesicle  the  growth  becomes 
more  rapid,  the  embryo  begins  to  rise  above  the  level  of  the  vesicle;  thus,  when, 
at  a  certain  point  of  the  surface  of  the  embryo,  a  steady  and  long-continued 
growth  occurs,  the  limb  appears,  gradually  lengthening  out,  and  enlarges  from  a 
small  bud  at  first  to  a  complete  arm  or  leg.  If  the  departure  takes  place  the 
other  way,  we  have  an  imagination  produced;  thus  for  every  hair  of  the  skin  and 
for  every  gland  of  the  intestine  there  is  a  separate  center  of  growth. 

The  causes  of  the  unequal  growths  are  unknown.  We  have  not  even  an 
hypothesis  to  offer  as  to  why  one  group  of  cells  multiplies  or  expands  faster  than 
another  group  of  apparently  similar  cells  close  by  in  the  same  germ-layer.  It  is 
no  real  explanation  to  say  that  it  is  the  result  of  heredity,  for  that  leaves  us  as 
completely  in  the  dark  as  ever  as  to  the  physiological  factors  at  work  in  the  devel- 
oping individual. 

The    conception    that    the    development    of    an   animal    depends    fundamentally 


*  By    this    limitation    \vc    exclude'    the    mesenchyma    hut    not    the    epithelium. 


GERM-CELLS.  25 

upon  the  unequal  expansion  and  consequent  foldings  and  bendings  of  the  germ- 
yers  was  first  suggested  by  the  researches  of  C.  F.  Wolff  on  the  development  of 
the  intestine,  and  was  more  clearly  recognized  by  Pander,  who  definitely  asserted 
that  the  formation  of  the  embryo  is  effected  by  foldings  of  the  germ-layers,  and 
the  truth  of  Pander's  view  was  conclusively  demonstrated  by  C.  E.  von  Baer  in  1828. 
In  recent  times  His  has  studied  the  problem  very  intently,  and  in  his  memoir  on 
the  chick  discussed  it  minutely.  In  this  memoir  is  to  be  found  most  of  what 
little  we  know  of  this  aspect  of  embryological  mechanics. 

Germ-cells. 

Recent  investigations  have  made  it  probable  that  a  few  cells  are  set  apart  dur- 
ing the  period  of  segmentation  to  form  the  germ-cells.  Their  number  is  small; 
they  preserve  for  some  time  the  appearance  of  segmentation  spheres,  as  the  cells 
which  are  formed  during  the  segmentation  of  the  ovum  are  sometimes  called. 
They  multiply  very  slowly  during  the  earliest  stages  of  development.  A  great 
majority  of  the  cells  produced  during  segmentation  lose  the  character  of  segmen- 
tation spheres,  and  divide  rapidly  and  repeatedly.  They  are  termed  somatic  cells 
and  form  the  various  tissues  of  the  body.  The  germ-cells,  on  the  contrary,  seem 
to  multiply  very  slowly  and  never  to  become  very  numerous  in  the  embryo.  As 
they  multiply  they  separate  from  one  another  and  become  more  or  less  completely 
surrounded  by  tissue  cells.  They  pursue  their  development,  one  is  tempted  to  say, 
independently  of  tissue  formation  and  somewhat  like  foreign  members  of  the  body. 
We  put,  accordingly,  the  germ-cells  in  a  class  by  themselves  in  contrast  to  the 
body  or  somatic  cells. 

Our  actual  knowledge  of  the  history  of  the  germ-cells  is  very  incomplete.  The 
statements  just  made  about  them  are  based  on  observations  on  very  few  animals. 
Their  exact  origin  has  been  traced  only  in  five  vertebrates,  three  fishes,  the  teleosts 
Cymatogaster  and  Micrometrus,  the  elasmobranch  Squalus  acanthias,  and  in  the 
frog  and  turtle.  In  these  five  forms  the  germ-cells  arise  during  segmentation,  and 
remain  more  or  less  closely  together,  or  segregated,  during  the  earliest  stages. 
They  then  separate  from  one  another  and  gradually  migrate  into  the  epithelium, 
which  covers  the  anlage  of  the  genital  gland  and  which  thus  becomes  the  so-called 
"germinal  epithelium." 

The  most  accurate  information  we  have  refers  to  their  development  in  the  dog- 
fish. In  this  species  the  germ-cells  are  delaminated  from  the  entoderm  together 
with  other  cells  of  the  mesoderm,  and  cannot,  with  our  present  knowledge,  be  dis- 
tinguished from  other  mesodermic  cells.  They  soon,  however,  become  recognizable, 
because  while  the  majority  of  the  mesodermic  cells  are  passing  into  the  second 
stage  (compare  the  section  on  Mesenchyma,  page  89)  these  germ-cells  change  but 
little,  if  at  all,  so  that  they  can  be  recognized  as  something  distinct  from  the  neigh- 
boring cells.  For  a  short  time  they  are  found  gathered  into  twro  compact  groups 
(Fig.  i,  Germ-cells]  symmetrically  placed  in  the  extra-embryonic  region,  but  not  far 


26 


GENERAL  CONCEPTIONS. 


from  the  embryo.  The  cells  then  break  apart  from  one  another  and  gradually 
become  separated,  and  migrate  by  unknown  means,  first  over  the  wall  of  the  intes- 
tine, which  has  meanwhile  been  differentiated,  then  over  the  surface  of  the  mesen- 
tery into  the  anlage  of  the  genital  gland.  During  their  entire  migration  they  are 
lodged  in  the  mesothelium,  and  when  they  have  reached  their  final  destination  they 
are  still  in  the  mesothelium  of  the  genital  anlage,  where  they  remain  until  finally 
differentiated  in  the  adult.  The  epithelium,  with  the  germ-cells  in  their  definite 
position  in  it,  is  called  the  germinal  epithelium  (compare  page  25).  The  germinal 
epithelium  has  been  observed  in  all  vertebrates,  but  the  origin  of  the  germ-cells  in 


FIG.  i. — SECTION  ACROSS  THE  POSTERIOR  PART  OF  AN  EMBRYO  DOG-FISH  (SQUALUS  ACANTHIAS).     TRANSVERSE 

SERIES  463,  SECTION  147. 

Ect.  Ectoderm.     Ent,  Entoderm.     Md,  Medullary  tube.     Mes,  Mesoderm.     Nch,  Notochord.    x,  Cellular  strand 

connecting  the  germ-cell  cluster  with  the  yolk. 


mammals  is  entirely  unknown.  The  hypothesis  may  be  accepted,  that  they  arise  in  a 
manner  essentially  similar  to  that  known  in  the  dog-fish.  For  some  of  the  theories 
based  on  the  known  development  of  the  germ-cells,  see  page  28. 

The  existence  of  the  germinal  epithelium  has  long  been  known,  and  its  charac- 
teristics have  been  described  in  many  text-books.  The  germ-cells  in  the  germinal 
epithelium  are  known  also  by  the  names  of  "sex-cellsn  and  "primitive  ova."  The 
transformation  of  these  cells  into  true  ova  has  been  traced  in  a  great  many  forms, 
so  that  the  transformation  may  be  considered  as  demonstrated  conclusively  for  all 
vertebrate  animals.  It  is  further  believed  that  the  germ-cells  also  give  rise  to  the 
male  elements,  playing  in  the  formation  of  the  testes  a  role  similar  to  that  which 


SEX.  27 

they  play  in  the  ovary.  The  proof  that  the  germ-cells  are  the  exclusive  parents  of 
the  spermatozoa  is  difficult  to  obtain,  but  most  embryologists  regard  the  existing 
proof  as  sufficient. 

When  a  germ-cell  is  transformed  into  an  ovum,  it  undergoes  great  enlargement,. 
its  nucleus  is  modified,  the  protoplasm  is  changed  in  appearance  and  becomes  loaded 
with  yolk  granules,  and  over  the  surface  of  the  cell  appear  two  membranes,  an 
inner  very  thin  one,  called  the  vitelline  membrane,  and  an  outer  much  thicker 
one,  known  as  the  zona  pellucida.  (For  a  fuller  description  see  page  34.)  We 
thus  learn  that  the  germ-cells  preserve  their  resemblance  to  segmentation  spheres 
only  during  embryonic  life.  When  they  become  ova,  they  pass  through  a  series 
of  important  changes  in  their  organization.  Since  germ-cells  also  give  rise  to  the 
male  elements,  we  must  say  further  that  in  order  to  produce  those  elements  the 
germ-cells  pass  through  another  series  of  profound  changes. 

It  is  further  known  that  in  order  to  evolve  the  sexual  elements,  both  male 
and  female,  the  cell  which  is  to  produce  them  divides  twice,  and  in  a  special  man- 
ner, which  we  designate  by  the  term  "reduction  division."  This  process  is  de- 
scribed in  all  the  recent  text-books  of  cytology  and  histology.  It  does  not  fall 
within  the  scope  of  this  work,  which  deals  with  embryology  in  the  strict  sense  only. 

Sex. 

The  sex  of  an  individual  depends  primarily  upon  the  nature  of  the  sexual 
glands.  The  same  two,  right  and  left,  parts  produce  the  sexual  glands  in  all  verte- 
brates. Each  part  originally  is  a  limited  area  of  the  surface  of  the  cephalad  end 
of  the  urogenital  ridge  (compare  p.  6)  and  becomes  either  a  testis  or  an  ovary, 
and,  since  the  two  sides  develop  alike,  the  individual  is  wholly  male  or  female  as 
the  case  may  be. 

As  an  exceedingly  rare  anomaly,  lateral  hermaphroditism  has  been  recorded.  In 
this  anomaly  there  is  a  testis  upon  one  side,  an  ovary  on  the  other.  This  is  the 
only  form  of  true  hermaphroditism  known  to  occur  in  the  amniota. 

Each  sex  is  further  distinguished  by  secondary  sexual  characteristics,  in  part  such 
as  are  immediately  concerned  with  reproduction,  like  the  uterus,  mammary  glands, 
vas  deferens,  etc.,  in  part  such  as  are  less  directly  connected  with  reproduction, 
such  as  size,  distribution  of  hair,  etc.  In  the  course  of  development  the  sexual 
glands  are  clearly  differentiated  before  the  secondary  sexual  characteristics  appear. 
Hence  arises  the  question,  have  the  glands  a  causal  relation  to  the  secondary 
characteristics  ? 

The  hormone  theory  is  the  only  one,  available  at  present,  to  explain  such  a 
causal  relationship.  It  is  known  that  various  glands  produce  a  so-called  internal 
secretion,  which  is  distributed  through  the  body  probably  by  the  medium  of  the 
blood  and  acts  upon  structures  quite  remote  from  the  organ  producing  the  secre- 
tion. Similar  chemical  products  arise  also  from  organs,  which  cannot  be  regarded 
as  glands  in  the  usual  sense.  All  of  these  secretions  or  products  have  received  the 


28  GENERAL  CONCEPTIONS. 

comprehensive  name  of  hormones  (Starling).  Now,  it  has  become  probable  that 
the  sexual  glands  produce  hormones,  which  exert  an  effect  upon  other  organs;  for 
example,  the  mammalian  corpus  luteum  is  believed  to  yield  a  hormone,  which  so 
affects  the  uterus  as  to  render  it  adapted  to  pregnancy.  The  hypothesis  accord- 
ingly naturally  suggests  itself  that  hormones  from  the  sexual  glands  occasion  the 
development  of  the  secondary  sexual  characteristics.  It  is  well  to  bear  in  mind  that 
our  .knowledge  of  hormones  is  still  meager,  and  that  the  suggested  hypothesis  may 
or  may  not  prove  valid. 

Concerning  the  cause  of  sex,  i.  e.,  why  one  individual  is  male  and  another 
female,  we  know  very  little.  Nothing  positive  as  to  .the  cause  of  sex  in  vertebrates 
is  known,  though  many  speculations  have  been  published.  In  certain  insects, 
however,  it  has  been  discovered  that  the  sexes  are  distinguished  by  the  females 
having  one  more  chromosome  in  each  cell  nucleus  than  the  males.*  This  difference 
is  explained  by  the  fact  that  there  are  two  kinds  of  spermatozoa,  one  of  which 
contains  an  extra  (accessory)  chromosome.  Those  ova  which  at  the  time  of  fer- 
tilization receive  an  accessory  chromosome  become  females,  those  that  do  not, 
males.  At  present  we  can  add  only  that  the  important  discoveries  mentioned  may 
furnish  the  clue  to  solve  the  problem  of  the  cause  of  sex. 

The  Theory  of  Heredity. 

We  owe  to  Moritz  Nussbaum  the  theory  of  germinal  continuity — the  only 
theory  of  heredity  which  seems  tenable  at  the  present  time.  According  to  this 
theory,  the  germ-cells  are  set  aside  during  the  segmentation  of  the  ovum  and  pre- 
serve the  essentially  undifferentiated  qualities  of  the  protoplasm  and  nucleus  of  the 
ovum,  from  the  division  of  which  they  arise.  Just  as  the  cells  formed  during  seg1 
mentation  are  capable  of  producing  the  various  tissues  of  the  body,  so  the  germ- 
cells  have  and  preserve  this  faculty.  If  we  term  the  material  of  the  original  ovum 
germ-plasm,  we  may  say  that  this  germ-plasm  gives  rise  to  the  various  tissue- 
forming  cells  which  make  up  the  body.  And  by  this  very  conversion  into  tissue 
cells  that  germ-plasm  is  changed,  and  is  no  longer,  as  we  have  learned  before, 
capable  of  the  full  range  of  development.  The  germ-cells,  on  the  contrary,  do  re- 
main so  capable  and  it  is  precisely  in  order  to  preserve  this  capacity  that  they  hold 
aloof  from  the  formation  of  the  body  tissues  and  pursue  their  own  independent 
career.  A  portion  of  the  germ-plasm  of  the  parent  ovum  is,  so  to  speak,  short- 
circuited  into  the  genital  elements  which  produce  the  offspring. 

If  we  accept  this  view,   we  are  forced  to  make  the    supplementary    hypothesis 
that   the   conspicuous   complicated    changes,  by  which    the    germ-cells   are   converted 
into   sexual   elements,   do   not   involve  differentiation   in   the   true   sense— i.  e.,  strictly " 
comparable    to    that  which  we  observe  in  the  somatic  cells.     Although  this  hypothe- 


*  In  other  insects  more  complicated  relations  have  been  discovered.     The  relation  of  the  chromosomes  to  sex 
appears  to  be  a  complicated  and  difficult  problem. 


THE  LAW  OF  RECAPITULATION.  29 

sis  seems  a  logical  necessity  of  the  theory  of  germinal  continuity,  we  cannot  at 
present  verify  it  by  any  observed  facts. 

The  only  other  theory  of  heredity  which  has  ever  been  seriously  considered  is 
that  of  pangenesis,  which  was  formulated  by  Darwin,  whose  words  I  quote:  "But 
besides  this  means  of  increase  I  assume  that  cells  before  their  conversion  into  com- 
pletely passive  or  'form-material'  throw  off  minute  granules  or  atoms,  which 
circulate  freely  throughout  the  system,  and  when  supplied  with  proper  nutriment 
multiply  by  self-division,  subsequently  becoming  developed  into  cells,  like  those  from 
which  they  were  derived.  These  granules,  for  the  sake  of  distinctness,  may  be 
called  cell-gemmules,  or  as  the  cellular  theory  is  not  fully  established,  simply 
gemmules.  They  are  supposed  to  be  transmitted  from  the  parents  to  the  off- 
spring, and  are  generally  developed  in  the  generation  which  immediately  succeeds, 
but  are  often  transmitted  in  a  dormant  state  during  many  generations,  and  are 
then  developed." 

Many  modifications  of  this  theory  have  been  proposed  by  speculative  writers, 
and  many  different  names  have  been  bestowed  upon  the  gemmules  of  Darwin  ac- 
cording to  the  fancy  of  each  author  and  the  particular  set  of  qualities  which  he 
attributed  to  these  imaginary  particles.  Such  views  attained  their  culmination  in 
the  set  of  elaborate  and  complicated  hypotheses  forming  the  doctrine  of  Weismann, 
or  so-called  Weismannism,  which  was  for  a  time  widely  and  actively  discussed.  All 
of  these  speculations  have  only  an  historical  interest,  having  proved  themselves, 
from  a  scientific  standpoint,  to  be  absolutely  barren. 

The  Law  of  Recapitulation. 

This  law,  as  commonly  formulated,  is  that  the  development  of  the  individual 
recapitulates  the  development  of  the  race,  or,  in  other  words,  the  ontogeny  recapit- 
ulates the  phylogeny.  This  way  of  stating  the  law  is  in  so  far  objectionable  that  it 
presents  the  theoretical  interpretation  of  the  law  rather  than  the  actual  generaliza- 
tion of  the  facts.  The  essential  datum  upon  which  the  law  is  based  is,  that  the 
embryo  of  a  given  animal  has  striking  morphological  resemblances  to  the  adult 
forms  of  lower  allied  types.  Since  the  theory  of  evolution  was  established  by  Dar- 
win this  resemblance  has  been  interpreted  as  due  to  the  inheritance  of  ancestral 
characters  appearing  in  the  embryo.  The  embryo  is  looked  upon  as  the  representa- 
tive of  the  actual  ancestor  by  modification  of  which  the  adult  form  was  evolved. 
It  is  further  assumed  that  the  change  of  the  embryo  into  the  adult  type  follows  the 
same  general  course  as  the  development  of  the  remote  ancestor  into  the  particular 
species  under  consideration.  Speaking  broadly,  this  interpretation  is  undoubtedly 
justifiable.  If  it  were  exactly  true,  it  would  be  necessary  only  to  know  the  em- 
bryology of  an  animal  in  order  to  establish  the  evolution  of  the  species.  Experience, 
however,  very  quickly  demonstrates  that  this  procedure  is  by  no  means  possible, 
because  the  embryo  is  not  a  correct  or  adequate  record  of  the  ancestral  type.  It  is 
inadequate  chiefly  for  three  reasons:  first,  because  the  embryo  has  necessities  of  its 


30  GENERAL  CONCEPTIONS. 

own,  and  in  the  course  of  evolution  embryos  acquire  special  peculiarities  by  which 
they  become  adapted  to  the  conditions  of  their  life.  Such  changes  in  organization 
do  not  correspond  to,  but  on  the  contrary  diverge  from,  the  inherited  ancestral 
traits,  and  in  so  far  as  they  are  present  they  mask  or  alter  those  structural  fea- 
tures of  the  embryo  which  represent  the  ancestral  record.  Second,  because  the  em- 
bryos consist  of  undifferentiated  cells  (compare  page  n).  Now,  the  adult  ancestors 
representing  lower  types  of  organization  of  course  had  differentiated  tissues,  which 
enabled  them  to  perform  the  functions  of  adult  life.  One  of  the  first  things  which 
will  impress  itself  upon  the  student  of  vertebrate  embryology  is,  that,  though  he  may 
find  at  the  proper  stage  in  the  embryo  the  organs  of  the  body  clearly  developed, 
yet,  owing  to  the  fact  that  they  consist  of  relatively  undifferentiated  cells,  they  are 
incapable,  in  large  part,  of  performing  the  functions  which  they  are  ultimately  to 
assume,  and  the  performance  of  which  is  the  very  object  of  their  development. 
This  change  in  histological  structure  brings  about  a  marked  unlikeness  of  the  em- 
bryo to  the  assumed  ancestral  type.  Third,  the  embryo  at  each  stage  of  its  de- 
velopment must  be  regarded  as  the  mechanical  cause  of  the  next  and  of  all  fol- 
lowing stages.  It  must  necessarily,  therefore,  have  in  itself  peculiarities  by  which 
it  is  distinguished  from  all  other  embryos.  It  is  impossible,  accordingly,  that  all 
embryos  should  be  alike.  It  is  only  necessary  for  the  student  to  compare  embryos 
of  various  vertebrates  one  with  another  to  satisfy  himself  that  they  have  conspicuous 
distinctive  characteristics.  When  our  knowledge  shall  have  grown  sufficintely, 
we  shall  be  able  to  classify  vertebrates  by  their  embryos  as  perfectly  as  or 
perhaps  even  more  perfectly  than  we  can  by  the  consideration  of  the  adult  forms. 
Every  embryo  is  modified  from  the  very  start  away  from  the  assumed  ancestral 
organization,  in  order  that  its  peculiarities  may  cause  it  mechanically  to  produce  the 
new  form  which  has  been  evolved. 

In  some  of  the  invertebrate  animals — as,  for  instance,  among  the  hydroids  and 
jellyfishes — the  law  of  recapitulation  can  be  much  more  easily  verified  than  in  the 
higher  forms  which  have  purely  embryonic  types  of  development.  From  what  has 
been  said,  it  will  be  recognized  that  the  likeness  of  the  embryo  to  the  adult  lower 
form  is  a  general  morphological  resemblance  only,  not  an  exact  one,  and  that  there- 
fore it  is  extremely  difficult  to  infer  from  the  embryonic  organization  what  the 
ancestral  type  was.  Hitherto  all  phylogenetic  inferences  drawn  by  embryologists 
have  been  largely  speculative  in  character,  and,  it  may  be  added,  have  been  more 
remarkable  for  their  number  and  variety  than  for  their  value. 

The  resemblance  between  embryos  and  lower  adult  forms  has  been  known  for 
a  century  past.  It  was  first  adequately  asserted  in  1811  by  J.  F.  Meckel  and 
since  then  has  been  constantly  discussed.  More,  perhaps,  was  done  to  emphasize 
it  by  Louis  Agassiz  than  by  anyone  else.  Von  Baer,  the  creator  of  modern 
scientific  embryology,  called  attention  in  1828  to  the  limitations  which  must  neces- 
sarily be  put  upon  Meckel's  generalization.  It  is  to  be  regretted  that  von  Baer's 
wise  thought  on  this  subject  has  not  been  'more  appreciated.  He  put  forth  four 


ARREST  OF  DEVELOPMENT.  31 

generalizations:  first,  that  which  is  common  to  a  large  group  of  animals  develops 
in  the  embryo  earlier  than  that  which  is  special;  second,  from  the  most  generalized 
stage  structures  less  generalized  are  developed,  and  so  on  until  finally  the  most 
special  appears;  third,  the  embryo  of  a  given  animal  form,  instead  of  passing 
through  the  other  given  forms,  separates  itself  from  them  more  and  more;  fourth, 
therefore,  essentially  the  embryo  of  the  higher  forms  is  never  like  a  lower  form, 
but  only  like  its  embryo.  The  first  to  point  out  the  possible  phylogenetic  signific- 
ance of  these  facts  with  perfect  clearness  was  Fritz  Miiller,  in  a  little  book  entitled 
"Fur  Darwin,"  published  in  1864.  Ernst  Haeckel  took  up  this  interpretation  and 
secured  wider  attention  for  it.  He  termed  the  law  of  recapitulation  the  "biogenetic 
law."* 

The  student  will  encounter  in  his  practical  study  many  illustrations  of  the 
resemblances  which  we  have  been  discussing,  so  that  it  is  unnecessary  here  to  do 
more  than  mention  a  few  for  the  purpose  of  illustration.  In  the  embryos  of 
birds  and  mammals  the  pharynx  forms  a  series  of  lateral  pouches  which  we  know 
as  the  gill-pouches,  and  which  develop  in  the  same  way  as,  resemble  strikingly, 
and  are  homologous  with,  the  gill-pouches  of  fishes,  which  in  the  fishes  give  rise 
to  the  so-called  gill-clefts.  The  heart  of  a  young  mammalian  embryo  is  a  simple 
tube  with  only  a  single  continuous  cavity  resembling  the  heart  of  the  lower  fishes. 
The  embryonic  kidney  or  Wolffian  body  of  man  resembles,  and  is  homologous  with, 
the  kidney  of  the  frog,  but  it  disappears  almost  completely  before  adult  life.  These 
few  examples  may  suffice. 

Arrest  of  Development. 

This  term  is  used  to  designate  not  the  normal,  but  the  abnormal,  cessation  of 
the  ontogenetic  process.  It  generally  implies  the  persistence  into  adult  life  of  an 
anatomical  condition,  normally  present  in  the  embryo,  which  is  typically  a  tem- 
porary though  essential  phase  of  development.  Usually  there  is  no  cessation  of 
the  histological  differentiation.  It  is  characteristic  of  these  anomalies  that  they  are 
more  or  less  definite. 

A  few  illustrations  may .  render  the  matter  clear.  The  palate  is  formed  as 
two  shelves,  which  grow  until  they  meet  in  the  median  line;  sometimes  they  fail 
to  meet  and  then  the  adult  has  a  "cleft  palate"  the  tissues  of  which,  however, 
are  as  fully  differentiated  as  those  of  the  normal  palate.  In  the  young  embryo  a 
short  blood-vessel  (i.e.,  dorsal  part  of  the  left  last  aortic  arch)  connects  the  pulmo- 
nary artery  with  the  dorsal  aorta  (compare  page  101),  but  it  later  becomes  occluded 
and  finally  obliterated.  Occasionally,  however,  it  persists  as  an  open  vessel,  which 
is  termed  the  "ductus  arteriosus"  in  the  adult.  It  may  grow  in  size  and  its  walls 
become  fully  differentiated  like  those  of  an  aorta.  The  external  genitalia  of  the 
male  may  be  arrested  in  their  development,  though  they  continue  in  such  cases 

*  "  Biogenetisches  Grundgesetz." 


32  GENERAL  CONCEPTIONS. 

their  growth  and  histogenesis.  There  results  a  "pseudo-hermaphrodite"  a  true  male 
with  external  organs  of  the  female  type.  In  such  cases  the  individual  usually 
presents  other  female  characteristics,  such  as  a  wide  pelvis  and  enlarged  mammae. 

The  variations  in  adult  structure  due  to  arrest  of  development  are  the  most 
frequent,  important,  and  significant  with  which  the  student  of  anatomy  has  to  deal. 
It  is  obvious  that  the  possible  variations  are  limited  to  anatomical  conditions,  which 
actually  occur  in  normal  embryos. 


CHAPTER  II. 
THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

The  Spermatozoon. 

The  spermatozoa  of  mammals  are  filaments  consisting  of  a  short  thick  end 
called  the  head,  and  a  very  long,  delicate  thread  called  the  tail.  They  are  of 
minute  size  as  compared  with  the  ovum.  The  head  varies  greatly 
'in  shape  according  to  the  species.  It  contains  chromatin,  hence 
it  stains  darkly  with  those  histological  dyes  which  color  nuclei. 
The  tail  consists  of  three  parts:  first,  the  middle  piece,  which  is 
next  the  head,  is  short  and  the  thickest  of  the  three  parts,  contains 
an  axial  thread,  and  probably  always  has  a  very  fine  spiral  thread 
running  round  it;  second,  the  main  piece,  which  is  the  longest 
part  of  the  tail;  and,  third,  the  end  piece,  which  is  not  more 
than  a  line,  even  as  seen  with  very  high  microscopic  powers. 

The  Human  Spermatozoon. — The  human  spermatozoon  is  0.055 
mm.  long — the  head  being  0.005  mm.,  the  tail  0.050,  and  the 
middle  piece  0.009.  It  is  shown  in  two  views  in  figure  2.  The 
head  is  flattened  and  pointed.  Seen  from  the  flat  side,  it  appears 
oval  (Fig.  2,  A)  with  the  front  end  generally  tapering  a  little,  but 
never  pointed.  The  anterior  half  or  two  thirds  has  a  brighter 
and  more  transparent  part.  Seen  on  edge  (Fig.  2,  B),  the 
head  has  a  pointed  form  with  a  posterior,  thicker,  round  dark 
part.  By  adjustment  of  the  focus  it  can  be  ascertained :  that  the 
sides  near  the  point  are  depressed.  Some  writers  maintain  that 
there  is  a  special  tip  projecting  from  the  head  as  a  cylinder 
thread,  with  a  hook  at  its  end.  The  middle  piece,  mi,  is  directly 
united  with  the  head  by  a  transverse  joint.  It  is  cylindrical  and 
about  as  long  as,  or  a  little  longer  than,  the  head.  Its  surface  is 
often  granular  or  rough,  and  there  cling  to  it  a  few  shreds  of 
protoplasm.  It  has  a  spiral  thread,  which  is  easily  overlooked 
on  account  of  its  extreme  fineness.  The  main  piece,  m,  of  the 
tail  is  about  half  as  thick  as  the  middle  piece.  It  gradually 
tapers  and  ends  abruptly  at  the  beginning  of  the  still  finer  and  very  short  end 
piece,  e.  The  tail  probably  contains  an  axial  thread,  as  has  been  observed  in  other 
3  33 


FIG.  2. —  HUMAN 
SPERMATOZOA. 

A, Complete  sperma- 
tozoon. B,  Head 
seen  from  the  side. 
C,  Extremity  of 
the  tail,  h,  Head. 
mi,  Middle  piece. 
m,  Main  piece,  e, 
End  piece.  A 11 
highly  magnified . 
— (After  Retzius.) 


34  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

mammals.      The  head  probably  contains  a  minute  body   representing  a  centrosome, 
although  it  has  not  yet  been  satisfactorily  demonstrated  in  man. 

The  spermatozoa,  when  free  in  the  fluids  in  which  they  normally  occur,  are 
capable  of  active  locomotion.  This  is  achieved  by  means  of  the  tail,  which  acts  as 
the  swimming  organ  by  vibratory  undulations  which  drive  the  spermatozoon  along, 
head  foremost.  The  tail  has  often  been  compared  to  the  flagellum  which  serves 
as  the  locomotive  organ  for  many  of  the  unicellular  organisms. 

The  Fully  Grown  Ovum  Before  Maturation. 

The  structure  to  be  here  described  is  not  the  true  sexual  element,  but  is  only 
the  modified  germ-cell  which  has  accomplished  its  period  of  growth  and  is  ready 
to  be  transformed  into  the  genuine  female  sexual  element.  This  transformation  is 
called  the  maturation,  and  is  accomplished  essentially  by  the  expulsion  of  the  so- 
called  polar  granules.  The  full-grown  mammalian  ovum  is  found  in  the  ovary  in 
the  center  of  the  discus  proligerus  of  the  Graafian  follicle.  It  measures  usually 
from  o.io  to  0.15  mm.  in  diameter.  It  is  approximately  spherical.  In  some  cases 
observers  have  found  a  very  delicate  vitelline  membrane  covering  the  protoplasm. 
Others  have  failed  to  observe  this.  Outside  there  is  a  thick  envelope  measuring 
from  0.02  to  0.03  mm.  in  diameter  and  known  as  the  zona  pellucida  or  radiata. 
Against  the  outside  of  the  zona  rest  the  cells  of  the  discus  proligerus  which  consti- 
tute the  so-called  "corona  radiata."  The  nucleus  is  large,  spherical,  contains  a 
distinct  nucleolus,  and  always  occupies  an  eccentric  position.*  The  protoplasm  of 
the  cell  is  large  in  amount,  granular  in  appearance,  forms  a  distinct  reticulum, 
and  contains  a  larger  or  smaller  number  of  yolk  granules  which  vary  considerably  in 
character,  size,  and  distribution  in  different  mammals.  They  are  usually  more  or 
less  concentrated  in  the  central  portion  of  the  ovum,  leaving  the  outer  portion, 
known  as  the  protoplasmic  zone,  more  or  less  free. 

The  Human  Ovum. — The  full-grown  human  ovum  is  distinguished  among 
mammalian  ova  for  the  clear  development  and  ready  visibility  of  all  its  parts,  a 
peculiarity  due  chiefly  to  the  small  amount  of  the  yolk  and  the  fewness  of  the 
fat  granules  it  contains.  Figure  3  represents  an  ovum  from  a  woman  of  thirty  years. 
The  specimen  was  obtained  by  ovariotomy,  examined  and  drawn  in  the  fresh  state, 
being  in  the  liquor  folliculi.  The  specimen  gave  the  following  measures:  The 
diameter  of  the  whole  ovum,  including  the  zona  radiata,  165-170;*;  thickness  of 
zona,  20-34;*;  perivitelline  fissure,  1.3/1;  the  clear  outer  zone  of  the  yolk,  4-6;*; 
the  protoplasmic  zone,  10-21;*;  the  zone  of  yolk  granules,  82-87;*;  nucleus,  25-27;*. 

The  corona  radiata,  cor.r,  consists  of  elongated  radiating  cells  with  rounded 
ends  and  oval  nuclei.  The  zona  pellucida  shows  a  distinct  radial  striation.  This 
is  probably  due  to  the  presence  of  minute  canals  running  through  the  zona.  The 
ovum  proper  is  separated  from  the  zona  by  a  narrow  fissure,  the  perivitelline  space, 


*The  nucleus  was  formerly  termed  "germinal  vesicle";  the  nucleolus,  " germinal  spot." 


OWL  AT  ION. 


35 


within  which  it  lies  free  and  loose.  Hence  when  a  fresh  specimen  is  examined, 
the  same  side  of  the  ovum,  that  containing  the  nucleus  and  which  is  the  lightest 
part,  is  always  found  uppermost.  In  this  ovum  no  vitelline  membrane  was  ob- 
served. The  body  of  the  ovum  may  be  divided  into  an  inner  kernel  containing 
the  yolk  granules,  and  an  outer  protoplasmal  zone,  of  which  the  very  thin  outer- 
most layer  is  clear  and  therefore  more  or  less  differentiated  from  the  broader,  deeper 
layer,  which  is  granular  and  constitutes  most  of  the  zone,  PL  The  yolk  grains 
are  i ,«  or  less  in  diameter.  They  are  highly  refringent  and  of  various  kinds. 
Their  characteristics  have  not  yet  been 
accurately  investigated.  The  nucleus  is  nearly 
spherical  and  has  a  conspicuous  nucleolus. 
In  fresh  specimens  the  nucleolus  shows 
amoeboid  movements,  even  at  ordinary 
summer  temperatures,  for  several  hours  after 
removal  from  the  ovary.  It  is  .only  in 
hardened  specimens  that  the  reticulum  of 
the  nucleus  can  be  clearly  observed. 


PL 


Nu. 


MATURATION. 

cor.r,  Part  of  corona  radiata.  Z,  Zona  pellucida. 
PI,  Protoplasm.  Y,  Yolk.  Nu,  Nucleus. — 
(After  W.  Nagel.) 


Ovulation. 

The  discharge  of  the  ovum  from  the 
ovary  is  called  ovulation.  It  results  from 
structural  changes  in  the  Graafian  follicle, 
and  these  changes  continue  after  the  de-  FIG.  3. — FULL-GROWN  HUMAN  OVUM  BEFORE 
parture  of  the  ovum,  transforming  the 
Graafian  follicle  into  a  so-called  corpus 
luteum.  The  exact  history  of  these  changes 
does  not  fall  within  the  scope  of  this  work. 

The  essential  steps  in  the  process  are  the  growth  of  the  Graafian  follicle  and 
the  thinning  of  its  wall  at  a  point  at  the  surface  of  the  ovary.  The  thin  part 
is  called  the  stigma.  This  breaks  through  and  establishes  an  opening  by  which 
the  ovum,  surrounded  by  the  corona  radiata,  together  with  the  liquor  of  the  follicle, 
can  escape  into  the  periovarial  chamber,  whence  it  makes  its  way  into  the  Fallo- 
pian tube.  The  growth  of  tissue  in  the  walls  of  the  collapsed  Graafian  follicle  fills 
up  the  space  of  the  same,  constituting  a  mass  which  is  known  as  the  corpus  luteum 
on  account  of  its  yellow  color.  The  most  characteristic  elements  of  this  structure 
are  the  large  cells  which  contain  the  pigment.  Each  cell  has  a  rounded  nucleus 
and  a  large  protoplasmic  body,  which  is  also  more  or  less  rounded  in  form.  The 
lutein  granules  are  in  these  cells.  The  function  of  the  corpus  luteum  was  long 
entirely  unknown.  Recently  the  theory  has  been  suggested  by  Born  that  these 
cells  exert  an  influence  upon  the  uterus  by  which  it  is  prepared  to  receive  the 
ovum.  This  influence  may  be  suggested  to  act  by  means  of  a  chemical  substance 
.(hormone)  produced  by  the  lutein  cells  and  added  to  the  blood,  which  then  affects 


36  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

the  uterus.     There  are  some  experimental  observations  tending  to  prove  the  correct- 
ness of  this  theory. 

The  brilliant  color  of  the  corpus  luteum  is  especially  characteristic  of  man,  and 
has  determined  the  name  of  the  structure.  In  sheep  the  pigment  is  pale  brown, 
in  the  cow  dark  orange,  in  the  mouse  brick-red,  in  the  rabbit  and  pig  flesh-color. 

The  Maturation  of  the  Ovum. 

Maturation  is  the  term  applied  to  the  series  of  changes  by  which  the  fully 
grown  egg-cell  is  transformed  into  a  true  female  sexual  element.  Viewed  externally 
in  the  living  ovum,  the  process  manifests  itself  chiefly  by  the  separating  off  of  two 
small  bodies  of  protoplasm,  each  of  which  contains  some  nuclear  material.  These 
small  bodies  are  generally  known  by  the  name  of  polar  globules.  They  take  no 
further  part  in  the  development,  ultimately  disintegrate,  and  are  lost.  The  remain- 
ing ovum  is  capable  of  impregnation.  It  is  now  known  that  the  production  of 
the  polar  globules  is  the  result  of  a  special  form  of  cell  division,  which  we  term 
the  "reduction  division."  When  the  first  polar  globule  is  formed,  the  egg-cell 
divides  into  one  very  large  cell  and  a  second  very  small  one.  When  the  second 
polar  globule  is  formed,  the  larger  of  the  cells  again  divides,  producing  a  second 
small  cell  and  a  new  large  one.  This  large  one  is  the  true  female  element. 

Maturation  as  a  general  process  may  be  described  as  follows.  For  figures  and 
detailed  accounts  of  the  process  in  the  mouse,  see  Chapter  V.  When  an  ovum  is 
about  to  mature,  its  nucleus  moves  nearer  that  point  of  the  surface  which  may  be 
regarded  as  the  center  of  the  so-called  animal  pole,  the  region  of  the  ovum,  which 
contains  most  of  the  protoplasm  and  less  of  the  yolk  material.  During  the  migra- 
tion of  the  nucleus,  the  cell  as  a  whole  usually  contracts  so  that  a  space  appears 
between  it  and  the  zona  radiata.  Concerning  the  force  that  moves  the  nucleus  we 
have  no  definite  knowledge.  When  near  the  surface,  the  nucleus  as  such  dis- 
appears. Older  writers  supposed  that  it  was  lost  altogether,  but  we  now  know 
that  the  disappearance  of  the  nucleus  is  only  apparent,  not  actual,  being  in  reality 
a  metamorphosis.  It  is  probable  that  the  first  step  is  the  discharge  of  the  nuclear 
fluid  into  the  surrounding  protoplasm,  causing  the  nucleus  to  become  more  or  less 
shriveled.  The  second  step  is  the  dissolution  of  the  membrane  of  the  nucleus  so 
that  the  nuclear  contents  are  brought  into  direct  contact  and  partly  mixed 
with  the  protoplasm  of  the  cells.  The  third  step,  which  in  time  more  or  less 
accompanies  the  second,  is  the  gathering  of  the  chromatin  of  the  nucleus  into  a 
definite  number  of  separate  granules  or  chromosomes  (tetrads').  These  chromosomes 
are  always  conspicuous  and  are  larger  than  those  formed  during  ordinary  cell 
division.  Their  number  is  also  highly  characteristic.  As  is  now  well  known,  there 
appear  during  the  process  of  indirect  cell  division  a  fairly  definite  number  of  chro- 
mosomes, a  number  which  is  characteristic  for  each  species.  In  numerous  cases 
it  has  been  observed  that  the  number  of  chromosomes  in  the  maturing  egg-cell  is 
exactly  one  half  of  that  found  during  the  ordinary  cell  divisions  of  the*  same  species.. 


THE  MATURATION  OF  THE  OVUM.  37 

The  chromatin  granules  lie  at  first  irregularly.  Fourthly,  there  arises  a  characteristic 
spindle  figure  such  as  accompanies  mitosis.  The  chromatin  forms  an  equatorial 
plate,  each  granule  being  associated  with  one  of  the  spindle  threads.  The  shape 
of  the  spindle  varies,  as  does  also  the  distribution  of  the  granules  of  the  equatorial 
plate.  In  guinea-pigs  the  ends  of  the  spindle  are  pointed  and  the  threads  are 
straight,  the  outline  of  the  spindle  being  like  a  diamond;  in  the  bat  the  spindles 
are  barrel-shaped  and  the  threads  are  curved.  In  many  cases  it  is  known,  and  it 
may  be  found  to  be  true  of  all  cases,  that  the  axis  of  the  spindle  is  at  right 
angles  to  the  radius  of  the  ovum.  The  nuclear  spindle  now  changes  its  position, 
becomes  first  oblique,  and  then  r.adial.  One  end  of  the  spindle  lies  close  to  the 
surface  of  the  ovum.  The  first  step  is  the  division  proper.  The  spindle,  driven 
by  an  undiscovered  power,  moves  centrifugally  until  it  is  partly  extruded  from  the 
egg.  The  projecting  end  is  enclosed  in  a  distinct  mass  of  protoplasm  which 
gradually  increases  and  soon  becomes  constricted  around  its  base.  The  fragments 
of  chromatin  have  each  divided  into  two  parts  (dyads),  and  one  half  of  each  frag- 
ment moves  toward  one  end,  and  the  other  half  toward  the  other  end  of  the 
spindle.  The  half  fragments  of  each  set  move  together,  hence  there  seem  to  be 
two  plates  within  the  spindle.  The  translation  of  the  groups  of  chromatin  con- 
tinues until  they  reach  the  end  of  ^the  spindle.  The  achromatic  threads  then 
break  through  in  the  middle,  and  thus  t\ie  original  nucleus,  or  at  least  a  part  of  it, 
has  been  divided.  There  are  now  two  masses  of  nuclear  substance,  one  in  the 
ovum,  the  other  in  the  polar  globule.  The  result  of  the  whole  process  is  the 
formation  of  two  cells  extremely  unequal  in  size,  and  each  containing  in  its  nuclear 
elements  half  the  number  of  chromosomes  characteristic  of  the  body-cells.  The 
number  of  chromosomes  has,  therefore,  been  reduced,  hence  the  term  reduction 
division.  It  will  be  noted  that  the  actual  reduction  in  the  number  of  chromosomes 
took  place  when  they  were  first  formed  in  the  maturing  ovum,  while  the  spindle  or 
mitotic  figure  was  developing. 

The  second  polar  globule  is  produced  a  short  time  after  the  first.  When  this 
occurs,  the  nuclear  remnants  in  the  ovum  do  not  reconstitute  themselves  into  a 
membranate  nucleus,  as  occurs  in  ordinary  cell  division,  but  they  change  directly 
into  a  second  spindle,  which  lies,  as  did  the  first,  within  the  protoplasm  of  the 
animal  pole.  This  second  spindle  occupies  an  oblique  position,  or  may  even  be 
parallel  with  the  surface.  But  it  also  soon  takes  up  a  radial  position  and  pro- 
duces a  second  polar  globule  in  similar  manner  to  the  first.  The  dyads  all  divide, 
and  the  ovum  receives  the  half  number  of  chromosomes,  each  of  which  represents 
the  fourth  part  of  a  tetrad  (double  chromosome).  The  second  globule  is  usually 
smaller  than  the  first. 

It  may  also  happen  that  the  first  polar  globule  may  itself  divide,  so  that  three 
polar  globules  appear. 

The  Formation  of  the  Female  Pro-nucleus. — The  nuclear  material  which  remains 
in  the  main  .ovum  after  the  separation  of  the  polar  globules  -is  known  as  the 


38  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

female  pro-nucleus.  The  nuclear  remnant  lies  close  to  the  animal  pole  and  in 
clear  protoplasm.  The  details  of  its  further  history  vary  according  to  the  species 
of  animal.  Three  tendencies  are  known  to  affect  the  pro-nucleus:  viz.,  to  move 
toward  the  central  position  in  the  ovum,  to  unite  with  the  male  pro-nucleus  as  soon 
as  that  is  formed  out  of  the  spermatozoon  which  enters  the  ovum,  and  to  assume 
the  character  of  a  membranate  nucleus.  As  the  time  of  the  formation  of  the  male 
pro-nucleus  is  variable,  the  other  tendencies  being  more  constant,  the  exact  history 
of  the  female  pro-nucleus  may  be  said  to  depend  principally  on  the  appearance 
of  the  male  pro-nucleus.  The  earlier  that  event,  the  less  does  the  female  pro- 
nucleus  move  centrifugally  and  the  less  does  it  assume  the  membranate  form. 
Even  among  mammals  there  is  variation. 

Time  of  Maturation. — The  time  when  the  polar  globules  are  formed  varies 
according  to  the  animal,  and  may  be  before  or  after  the  egg-cell  leaves  the  ovary. 
In  placental  mammals  maturation  always  begins,  so  far  as  known,  in  the  ovary, 
and  is  said  in  some  cases  to  be  completed  there.  But  in  other  cases  it  is  certainly 
completed  only  after  ovulation  or  when  the  ovum  has  passed  into  the  Fallopian  tube. 

Impregnation  of  the  Ovum. 

Impregnation  is  the  union  of 'the  male  and  female  elements  to  form  a  single 
new  cell,  capable  of  initiating  by  its  own  division  a  rapid  succession  of  generations 
of  descendent  cells.  The  process  of  union  is  commonly  called  the  entrance  of  the 
spermatozoon  into  the  ovum.  The  new  cell  is  called  the  impregnated  or  fertilized 
ovum.  The  process  of  fertilization  in  the  mouse  is  described  and  illustrated  in 
Chapter  V. 

In  all  multicellular  animals  impregnation  is  effected  by  three  successive  steps: 
(i)  The  bringing  together  of  the  male  and  female  elements;  (2)  the  entrance  of  the 
spermatozoon  into  the  ovum  and  the  formation  of  the  male  pro-nucleus;  (3)  fusion 
of  the  pro-nuclei  to  form  the  segmentation  nucleus. 

Meeting  of  the  Sexual  Elements.— In  all  amniota  the  seminal  fluid  is  transferred 
from  the  male  to  the  female  passages  during  coitus,  and  spermatozoa  are  thereafter, 
in  mammals,  found  in  the  uterus.  In  default  of  actual  knowledge  it  is  commonly 
believed  that  the  spermatozoa  make  their  way  by  their  own  motions  into  the  Fallo- 
pian tubes.  The  ovum,  meanwhile  (probably,  in  mammals,  while  completing  its 
maturation),  travels  down  the  tube.  The  meeting-point,  or  site  of  impregnation, 
in  placental  mammals  is  about  one-third  way  down  from  the  fimbria  to  the  uterus. 
The  exact  spot  is  remarkably  constant  for  each  species.  Nothing  is  known  by 
direct  observation  as  to  the  site  of  impregnation  in  man,  but  there  is  no  jeason  to 
suppose,  as  has  unfortunately  been  often  done,  that  the  site  is  either  variable  or 
essentially  different  from  that  in  other  mammals. 

The  Entrance  of  the  Spermatozoon  into  the  Ovum.— It  is  probable,  in  mammals 
at  least,  that  only  one  spermatozoon  normally  enters  the  yolk  of  the  ovum  and  ac- 
complishes its  fertilization.  It  has  been  observed  in  those  animals  in  which,  as  in 


IMPREGNATION  OF  THE  OVUM.  39 

the  rabbit,  there  is  formed  a  more  or  less  considerable  space  between  the  yolk  and 
the  zona  radiata,  that  a  number  of  spermatozoa  appear  in  this  space,  but  appar- 
ently only  one  actually  fuses  with  the  substance  of  the  ovum.  The  manner  in 
which  additional  spermatozoa  are  excluded,  after  the  first  has  entered,  is  still 'under 
discussion.  The  hypothesis  has  been  suggested  that  the  attractive  power  of  the 
ovum  is  annulled  or  weakened  by  the  formation  of  the  male  pro-nucleus  from  the 
spermatozoon  which  first  enters.  With  our  present  knowledge  the  assumption 
appears  unavoidable  that  the  ovum  exerts  a  specific  attraction  upon  spermatozoa 
of  the  same  animal  species.  Recent  authorities  incline  to  the  view  that  this  attrac- 
tion is  of  a  chemical  nature,  for  it  has  been  observed  that  certain  chemical  sub- 
stances may  attract  very  strongly  unicellular  organisms  capable  of  free  locomotion. 
The  phenomenon  is  called  chemotropism.  According  to  this  interpretation,  the 
attraction  of  the  ovum  for  the  spermatozoon  would  be  termed  chemotropic. 

At  the  time  of  fertilization  the  ovum  in  the  Fallopian  tube  is  surrounded  by 
a  number  of  spermatozoa;  in  the  case  of  the  rabbit,  perhaps  by  a  hundred,  more 
or  less.  They  are  all,  or  nearly  all,  in  active  motion,  for  the  most  part  pressing 
their  heads  against  the  zona  radiata.  Several  of  them  may  make  their  way  through 
the  zona  into  the  interior.  According  to  Hensen,  only  those  spermatozoa  which 
enter  the  zona  along  radial  lines  can  make  their  way  through.  Those  which  take 
oblique  courses  remain  caught  in  the  zona.  The  female  pro-nucleus  is  already 
present,  and  may  be  either  with  or  without  a  membrane,  according  to  the  species. 
A  single  spermatozoon  makes  its  way  into  the  yolk  proper,  passing  a  short  distance 
into  the  interior.  It  is  uncertain  whether  the  whole  tail  of  the  spermatozoon  enters 
the  ovum  or  not.  In  some  of  the  lower  vertebrates  and  in  other  animals  it  ap- 
pears to  do  so.  It  is  probably  always  the  case  that  at  least  the  main  piece  of  the 
tail  enters  the  yolk.  The  tail,  as  such,  very  soon  disappears,  while  the  head  of 
the  spermatozoon  enlarges,  probably  by  the  imbibition  of  fluid  from  the  surrounding 
yolk.  The  head  of  the  spermatozoon  is  rich  in  chromatin,  which  forms  a  series 
of  irregular  masses  as  the  head  enlarges,  producing  a  network  appearance,  and  is 
thus  converted  into  a  nucleus-like  body,  the  male  pro-nucleus.  At  the  same  time 
in  some  animals  the  growing  head  surrounds  itself  by  a  membrane. 

We  now  have  a  cell  which  contains  two  nucleus-like  bodies,  one  derived  from 
the  head  of  the  spermatozoon  and  the  other  from  the  nucleus  of  the  egg-cell.  They 
are  termed  respectively  the  male  and  female  pro-nucleus.  Each  pro-nucleus,  when 
it  first  appears,  is  small  and  gradually  enlarges,  probably  in  both  cases  by  the  im- 
bibition of  fluid.  The  relative  size  of  the  two  pro-nuclei  varies  considerably  in 
different  species,  and  is  probably  a  secondary  and  relatively  unimportant  relation. 
The  proportion  between  the  two  probably  depends  upon  the  time  when  the  male 
pro-nucleus  is  formed.  If  the  spermatozoon  enters  early,  while  the  female  pro- 
nucleus  is  forming,  it  may  make  a  pro-nucleus  as  large  as  that  from  the  egg- 
cell.  If,  on  the  other  hand,  the  spermatozoon  enters  late,  the  female  pro-nucleus 
enlarges,  acquires  a  start,  and  the  growing  male  pro-nucleus  is,  therefore,  smaller. 


40  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

Concerning  the  fate  of  the  middle  piece  of  the  spermatozoon  and  its  share 
in  the  fertilization  in  the  ovum  of  mammals,  we  possess  no  satisfactory  informa- 
tion. It  has  been  shown,  however,  in  other  animals  that  this  middle  piece 
produces  a  centrosome,  and  the  only  centrosome  which  appears  in  the  fertilized 
ovum.  The  theory  has  been  advanced  that  the  ovum,  after  its  maturation,  has 
no  centrosome,  that  a  centrosome  is  always  brought  into  the  ovum  by  the  sper- 
matozoon in  the  manner  just  indicated.  If  we  regard  the  centrosome  as  a  perma- 
nent cell  element,  then  we  must  further  interpret  the  addition  of  the  male  centro- 
some as  one  of  the  most  important  phenomena  of  fertilization.  Whether  this 
hypothesis  is  correct  or  not,  we  are  unable  at  present  to  decide. 


FIG.  4. — OVUM  OF  A  WORM  (SAGITTA)  WITH  Two      FIG.'  5. — OVUM  OF  A  RABBIT,  SEVENTEEN  HOURS 
PRO-NUCLEI.     AROUND  EACH  PRO-NUCLEUS  is  AFTER  COITUS,  WITH  THE  PRO-NUCLEI  ABOUT 

SHOWN  THE  ASTER. — (After  O.  JETertwig.)  TO  CONJUGATE. 

p.g,  Polar  globules. — (After  Rein.) 

Astral  figures  play  a  conspicuous  part  in  the  phenomenon  of  fertilization  in 
many  animals.  Astral  figures  are  produced  in  the  protoplasm  of  the  ovum  by  its 
assuming  a  special  radiating  structure.  Astral  figures  may  appear  around  both 
the  male  and  female  pro-nuclei  (Fig.  4).  In  other  cases  the  astral  figure  arises 
only  in  association  with  the  head  of  the  spermatozoon  or  male  pro-nucleus.  In 
mammals,  so  far  as  known,  no  astral  figures  are  developed  about  either  of  the 
pro-nuclei.  There  is  a  clear  space  in  the  protoplasm  around  each  nucleus,  and 
such  a  clear  space  has  often  been  noted  also  when  the  astral  figure  is  present.  It 
may  possibly  be  interpreted  as  a  rudimentary  aster  or  center  of  astral  formation. 

The  two  pro-nuclei  usually  lie  at  some  distance  from  one  another.  As  soon 
as  they  are  formed,  or  perhaps  when  they  are  fully  differentiated,  they  tend  to 
move  toward  one  another  and  toward  the  center  of  the  ovum.  Concerning  the 
path  of  the  male  pro-nucleus  we  possess  interesting  information  from  the  study  of 
the  ova  of  the  frog  and  axolotl.  In  these  ova  the  spermatozoon  leaves  a  trail  of 
pigment,  which  consists  of  two  limbs,  one  passing  through  the  cortical  layer  of  the 
ovum  nearly  perpendicular  to  the  surface,  and  the  other  forming  an  angle  with  the 
first  and  leading  directly  to  the  female  pro^-nucleus.  The  female  pro-nucleus  tends 


IMPREGNATION  OF  THE  OVUM.  41 

always  to  move  toward  a  central  position.  The  force  which  draws  the  pro-nuclei 
together  is  unknown.  The  hypothesis  that  this  force  is  chemotropic  has  met  with 
favor. 

The  Fusion  of  the  Pro-nuclei. — In  the  rabbit,  as  probably  in  all  mammals,  both 
pro-nuclei  lie  at  first  eccentrically,  but  both  move  toward  each  other  and  toward 
the  center,  meeting  when  the  central  position  is  attained.  As  they  near  one  an- 
other both  pro-nuclei  perform  active  amoeboid  movements.  After  they  meet  they 
still  continue  their  amoeboid  movements,  and  travel  together  to  the  center  of  the 
ovum  (Fig.  5).  One  of  the  pro-nuclei  assumes  a  crescent  shape  and  embraces  the 
other.  In  the  mouse  the  history  is  similar.  After  the  two  pro-nuclei  in  this  animal 
have  met,  they  remain  side  by  side,  but  they  are  without  membranes.  After  the  con- 
junction of  the  pro-nuclei  the  chromatin  threads  become  distinct  and  draw  closer 
together.  Between  them  appears  first  a  small  spot  or  centrosome  with  a  few  radi- 
ating lines  around  it  (Fig.  117).  From  the  centrosome  arises  a  spindle  of  achro- 
matic threads  (Fig.  118).  The  chromosomes,  both  male  and  female,  attach  them- 
selves to  the  spindle,  and  therewith  impregnation  is  completed  and  mitosis  of 
the  impregnated  ovum  initiated. 

It  is  now  believed  that  the  pro-nuclei  never  unite  to  form  a  distinct  mem- 
branate  nucleus,  the  so-called  segmentation  nucleus  of  earlier  writers,  but  that  the 
fusion  always  takes  place  during  the  absence  of  the  membranes  of  the  pro-nuclei  by 
the  mingling  of  their  contents.  The  time  of  mingling,  however,  varies  as  regards 
the  formation  of  the  chromosomes.  It  may  take  place  before  or  after  the  chromo- 
somes are  developed.  When,  as  in  the  mouse,  the  chromosomes  appear  as  two 
distinct  groups,  it  is  possible  sometimes  to  determine  their  number.  In  the  mouse 
counting  is  difficult,  but  there  seems  little  doubt  that  each  pro-nucleus  forms 
twelve  chromosomes.  Hence  it  results  that  there  are  twenty-four  chromosomes  in  the 
segmentation  spindle.  This  number,  twenty-four,  so  far  as  has  been  determined,  is 
the  number  which  appears  during  later  stages  of  segmentation  and  in  all  subse- 
quent cell  divisions  of  this  animal.  It  is  believed  to  be  a  general  law  that  the  male 
and  female  pro-nuclei  each  contribute  the  same  number  of  chromosomes  to  the  seg- 
mentation spindle  except  in  those  cases  where  an  accessory  chromosome  is  inter- 
polated in  the  development.  This  number  is  identical  with  the  number  which  ap- 
pears during  the  reduction  divisions  which  lead  to  the  maturation  of  the  ovum  on 
the  one  hand  and  the  development  of  the  spermatozoon  on  the  other;  and, 
further,  the  number  is  one  half  the  number  of  chromosomes  which  appear  during 
ordinary  cell  divisions  of  the  species.  The  most  thorough  study  of  the  phenomenon 
which  has  yet  been  made  is  that  by  a  succession  of  able  investigators  upon  the 
large  nematode  Ascaris  megalocephala.  An  admirable  summary  of  the  process  of 
fertilization  in  Ascaris  has  been  given  by  Oscar  Hertwig.* 

*  "  Lehrbuch  der  Entwicklungsgeschichte,"  eighth  edition,  1906.  The  large  Ascaris  is  a  particularly 
favorable  object.  The  student  who  wishes  to  pursue  the  practical  study  of  impregnation  further  should  select 
this  form. 


42 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


Segmentation  of  the  Ovum. 

Shortly  after  the  entrance  of  the  spermatozoon  into  the  ovum  the  segmenta- 
tion spindle  is  developed  by  the  union  of  the  pro-nuclei,  as  described  in  the  pre- 
vious section.  This  spindle  leads  to  a  division  of  the  ovum  into  two  cells.  These 
cells  further  rapidly  divide.  As  stated  on  page  10,  these  early  cell  divisions  are 
called  the  segmentation  of  the  ovum. 

The  position  of  the  first  segmentation  spindle  is  always  eccentric,  and  appears 
to  be  approximately,  if  not  exactly,  the  same  as  that  of  the  egg-cell  nucleus  before 
maturation.  The  axis  of  the  spindle  varies  greatly  in  its  direction.  It  is  some- 
times at  right  angles  to  the  radius  of  the  ovum,  which  passes  through  the  polar 


FIG.   g. — OVUM   OF  A  RABBIT  OF  TWENTY-FOUR      FIG.   7. — OVUM  OF  A  SNAIL  (LIMAX  CAMPESTRIS) 


DURING    THE   FIRST   CLEAVAGE.      THE    ENVEL- 
OPES OF  THE  OVUM  ARE  NOT  DRAWN  IN.       X  2OO 

diams. — (After  E.  L.  Mark.) 


HOURS. 

The  first  cleavage  has  been  completed;  the  two  cells 
(segmentation  spheres)  are  appressed;  above  the 
cells  lie  the  polar  globules;  numerous  spermato- 
zoa lie  in  and  within  the  zona  pellucida. — 
(After  Coste.) 

globules,  but  it  is  more  usually  oblique  to  this  radius.  It  was  at  one  time  thought 
that  the  plane  of  division  was  always  at  right  angles  to  the  radius  of  the  extrusion 
of  the  polar  globules,  but  this  view  cannot  be  upheld.  After  the  ovum  has  divided 
into  two  cells,  the  polar  globules  lie  in  the  angle  between  these  two  cells  (Fig.  6), 
because  there  the  globules  find  room.  It  is  to  be  noted  that  the  globules  accom- 
modate themselves  to  the  segmentation  spheres,  and  that  the  formation  of  the 
spheres  is  not  accommodated  to  the  original  position  of  the  globules. 

The  degree  of  the  eccentricity  of  the  segmentation  spindle  varies  in  different 
ova,  chiefly  according  to  the  amount'  of  yolk;  the  greater  the  quantity  of  yolk  in 
the  ovum,  the  more  marked  is  the  eccentricity. 

The  actual  first  cell  division  (first  •  cleavage  or  first  segmentation)  of  a  mam- 
malian ovum  has  never  been  followed  by  direct  observation,  the  practical  diffi- 
culties not  having  hitherto  been  successfully  overcome.  Various  phases  of  the  di- 
vision have,  however,  been  seen,  and  the  internal  changes  have  been  studied  by 
means  of  sections.  It  accordingly  seems  expedient  to  interpolate  the  following  ac- 


SEGMENTATION  OF  THE  OVUM.  43 

count  of  the  external  appearances  of  the  first  segmentation  in  the  living  ovum  of 
the  snail,  Limax  campestris.  The  eggs  of  this  animal,  by  their  size  and  in  their 
mode  of  segmentation,  have  a  certain  resemblance  to  mammalian  ova.  The  fol- 
lowing description  is  taken  from  the  account  by  E.  L.  Mark,  published  in  1881;  it 
is  nearly  in  his  own  words: 

In  Limax,  after  impregnation,  the  region  of  the  segmentation  nucleus  remains 
more  clear,  but  all  that  can  be  distinguished  is  a  more  or  less  circular,  ill-defined 
area,  which  is  less  opaque  than  the  surrounding  portion  of  the  vitellus.  After  a 
few  moments  this  area  grows  less  distinct.  It  finally  appears  elongated.  Very  soon 
this  lengthening  results  in  two  light  spots,  which  are  inconspicuous  at  first,  but  which 
increase  in  size  and  distinctness,  and  presently  become  oval.  If  the  outline  of  the 
egg  be  carefully  watched,  it  is  now  seen  to  lengthen  gradually  in  a  direction  corre- 
sponding to  the  line  which  joins  the  spots.  As  the  latter  enlarges  the  lengthening 
of  the  ovum  increases,  though  not  very  conspicuously.  Soon  a  slight  flattening  of 
the  surface  appears  just  under  the  polar  globules;  the  flattening  changes  to  a  de- 
pression (Fig.  7),  which  grows  deeper  and  becomes  angular.  A  little  later  the  fur- 
row is  seen  to  have  extended  around  on  the  sides  of  the  yolk  as  a  shallow  de- 
pression, reaching  something  more  than  halfway  toward  the  vegetative  or  inferior 
pole,  and  in  four  or  five  minutes  after  its  appearance  the  depression  extends  com- 
pletely around  the  yolk.  This  annular  constriction  now  deepens  on  all  sides,  but 
most  rapidly  at  the  animal  pole;  as  it  deepens  it  becomes  narrower,  almost  a  fis- 
sure. By  the  further  deepening  of  the  constriction  on  all  sides  there  are  formed 
two  equal  masses  connected  by  only  a  slender  thread  of  protoplasm,  situated  nearer 
the  vegetative  than  the  animal  pole;  the  thread  soon  becomes  more  attenuated  and 
finally  parts.  The  first  cleavage  is  now  accomplished.  Both  segments  undergo 
changes  of  form;  they  approach  and  flatten  out  against  each  other,  and  after  a 
certain  time  themselves  divide. 

The  succeeding  cleavages  of  segmentation  need  to  be  followed  out  in  greater 
detail  than  yet  recorded.  In  many  cases  there  appear  to  be  three  cells  in  the 
next  stage,  because  one  of  the  two  primitive  segmentation  spheres  divides  sooner 
than  the  other.  The  more  commonly  received  view  is  that  four  cells  are  produced 
next,  but  it  may  very  well  be  that  there  is  really  a  three-cell  stage  preceding  the 
four-cell  stage  of  which  two  figures  are  presented.  The  first  of  these  (Fig.  8) 
represents  the  four-cell  stage  of  the  ovum  of  a  bat,  and  the  second  (Fig.  9)  repre- 
sents the  four-cell  stage  of  the  ovum  of  the  Virginian  opossum.  That  of  the  bat 
resembles  the  picture  which  we  obtained  from  a  number  of  animals,  such  as  the 
rabbit,  the  guinea-pig,  the  dog,  and  others.  That  of  the  opossum  differs  so  much 
from  anything  known  in  other  mammals  that  it  may  be  questioned  whether  it  is 
entirely  normal.  In  the  mouse  the  zona  is  much  thinner  and  assumes  an  irregular 
form,  adapting  itself  to  the  pressure  of  the  single  spheres. 

After  the  four-cell  stage,  the  segmentation  proceeds  apparently  with  considerable 
irregularity,  but  we  are  soon  able  to  see  that  the  cells  are  grouping  themselves 


44 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


into  an  uninterrupted  external  layer  and  an  internal  accumulation  of  cells.  The 
outer  layer  is  in  contact,  or  nearly  in  contact,  with  the  zona  radiata,  and  may, 
therefore,  be  termed  the  subzonal  layer  (Fig.  n,  5.2).*  The  inner  accumulation  of 
cells  is  designated  as  the  inner  mass,  i.m.  Figure  10  represents  a  rabbit  ovum  of 
about  seventy  hours,  according  to  the  observations  of  van  Beneden.  He  represents 
the  subzonal  layer,  EC,  as  interrupted  at  one  point,  where  one  of  the  cells  of  the 
inner  mass,  i.m,  is  exposed.  It  is  probable,  however,  that  -  van  Beneden  is  in 
error  in  regard  to  this,  and  that  the  subzonal  layer  is  really  continuous.  In  the 


FIG.  8. — OVUM  OF  A  BAT  (VESPERTILIO  MURINA) 
WITH  FOUR  SEGMENTATION  SPHERES. — (After 
van  Beneden  and  Julin.) 


X300 


FIG.  9. — OVUM  OF  A  VIRGINIAN  OPOSSUM,  WITH 

FOUR  SEGMENTS. 

.p.g}  Polar  globules,   x,  Coagulated  material,    z,  Zona 
pellucida. — (After  Emil  Selenka.) 


next  stage  (Fig.  n)  we  find  that  the  ovum  has  become  larger  by  the  appearance 
of  a  cavity  in  its  interior.  This  cavity  appears  between  the  inner  mass,  i.m,  and 
the  subzonal  layer,  but  at  one  side  the  inner  mass  remains  adherent  to,  and  closely 
connected  with,  the  subzonal  layer.  We  now  have  reached  the  stage  in  which  the 
developing  ovum  may  be  designated  as  the  blastodermic  vesicle. 

As  to  the  interpretation  of  the  parts,  it  is  probable  that  the  subzonal  layer 
is  ectoderm,  and  that  the  central  cells  of  the  inner  mass  are  also  ectodermal  and 
share  in  forming  the  embryonic  shield,  and  finally  that  the  superficial  cells  of  the 
inner  mass  (i.e.,  those  next  the  cavity  of  the  vesicle)  are  entodermal.  At  the  stage 
we  have  now  reached  the  blastodermic  vesicle  has  a  large  part  of  its  walls  formed 
by  the  subzonal  layer  only,  so  that  we  call  this  the  stage  of  the  one-layered  blasto- 
dermic vesicle. 

*  The  subzonal  layer  is  termed  trophoblast  by  A.  A.  W.  Hubrecht,  and  is  held  by  him  to  be  a  special  embryonic 
structure,  developed  in  order  fo  establish  special  relations  between  the  developing  ovum  and  the  walls  of  the  uterus 
to  secure  the  nutrition  of  the  former.  It  has  seemed  best  to  present  a  purely  objective  account  of  the  facts  without 
entering  into  a  discussion  of  the  very  interesting  interpretations  proposed  by  Hubrecht. 


THE  BLASTODERM  1C  VESICLE. 


45 


Arrival  in  the  Uterus. — During  the  stages  described  the  ovum  travels  along  the 
Fallopian  tube  and  reaches  the  uterus  in  an  early  phase  of  the  stage  which  we 
designate  as  the  •  blastodermic  vesicle.  The  transit  requires  about  eighty  hours  in 
the  mouse,  about  five  days  in  the  opossum,  four  days  in  the  rabbit,  and  from  eight  to 
ten  days  in  the  dog.  The  time  necessary  in  man  is  unknown.  It  may  be  sup- 
posed to  be  about  one  week. 

Pro-chorion. — The  ovum  in  many  mammals  becomes  surrounded  by  a  gelatinous 
covering,  which  is  secreted  by  the  glands  of  t  the  uterus.  It  may  be  compared 


km. 


FIG.    10. — RABBIT'S   OVUM    or    ABOUT    SEVENTY 

HOURS. 

EC,  Outer  layer,     i.m,  Inner  mass  of  cells.     Z,  Zona 
pellucida. — (After  E.  van  Beneden.) 


S>z. 


FIG.    ii. — YOUNG   BLASTODERMIC  VESICLE   OF   A 

MOLE. 

i.m,  Inner  mass  of  cells.     s.z,  Outer  or  subzonal 
layer,     z,  Zona  pellucida. — (After  W.  Heape.) 


to  the  white  of  the  bird's  egg.  In  _the  rabbit  this  envelope  becomes  enormously 
thick  about  the  blastodermic  vesicle  and  in  other  rodents  is  voluminous.  In  the 
dog  it  is  less  developed,  but  presents  the  further  peculiarity  that  the  secretion  in 
the  tubular  glands  may  be  hardened  in  connection  with  the  envelope  itself,  which, 
therefore,  appears,  when  the  ovum  is  removed  from  the  uterus,  to  be  studded  over 
with  fine  threads  resembling  villi.  The  gelatinous  envelope  has  been  termed  by 
Hensen  the  pro-chorion.  The  thread-like  projections  seen  in  the  dog  were  taken 
by  Bischoff  for  true  villi,  and  they  have  sometimes  been  referred  to  as  the  pro- 
chorionic  villi.  The  term  pro-chorion  has  been  applied  to  other  structures,  as, 
for  instance,  to  the  subzonal  layer  of  the  blastodermic  vesicle.  The  student  needs 
to  be  warned  against  confusing  the  term  pro-chorion  in  its  various  applications. 

The  Blastodermic  Vesicle. 

The  blastodermic  vesicle  always  consists  at  first  of  the  subzonal  layer  and  an 

inner   cell   mass    attached    at    one    point    to    the  subzonal   layer,    and    has    a   cavity 

between  the  inner  mass  and  the  subzonal  layer;  the  vesicle  itself  is  always  enclosed 


46 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


in  the  zona  radiata.  The  variations  offered  in  different  mammals  are  so  great  that 
a  description  less  general  than  that  given  would  hardly  be  applicable,  even  to  the 
placental  mammals. 

The  next  step  in  development  is  the  production  of  a  complete  second  layer 
out  of  the  cells  of  the  inner  mass.  This  layer  extends  completely  around  the 
vesicle  and  lies  close  against  the  subzonal  layer,  and  encloses  the  main  cavity  of 

the  vesicle.  The  way  in  which  this  inner 
vesicular  layer  is  developed  varies  greatly. 
In  the  hedgehog  it  appears  very  preco- 
ciously, while  the  blastodermic  vesicle  is 
very  small,  and  afterward  it  expands  rapidly, 
while  the  vesicle  as  a  whole  is  growing.  In 
the  rabbit  and  in  the  mole  it  is  formed 
much  later,  and  the  one-layered  vesicle  ex- 
pands to  a  considerable  diameter  before  the 
inner  mass  begins  to  spread  out.  The  strik- 
ing changes  through  which  the  inner  mass 
passes  in  the  mole  are  illustrated  in  figure  12. 
It  forms  at  first  a  small  globe,  A.  The 
inner  mass  subsequently  flattens  out,  becom- 
ing lens-shaped,  thinner,  and  larger  in 
area,  B.  It  continues  spreading  laterally 
and  separates  into  three  layers.  The  two 
outer  layers  enter  into  the  formation  of  the 
true  ectoderm,  C.  In  the  rabbit,  and  per- 
haps in  the  mole,  the  outer  of  the  two 
FIG.  12.— SECTIONS  THROUGH  THE  INNER  MASS  OF  layers  is  temporary  only  in  existence.  In 
BLASTODERMIC  VESICLES  OF  THE  MOLE  AT  some  rodents  it  acquires  a  very  great  de- 
velopment and  leads  to  the  curious  phe- 
nomenon known  as  the  inversion  of  the 
germ-layers.  The  innermost  of  the  layers, 
Ent,  grows  at  its  edges,  and  its  cells  spread 

out  gradually  farther  and  farther  under  the  subzonal  layer  until  they  extend  com- 
pletely around  the  vesicle  and  form,  by  meeting  at  the  opposite  pole  of  the  ovum, 
a  closed,  vesicle.  Very  similar  is  the  process  in  the  rabbit.  The  cells  at  the  expand- 
ing edge  of  the  inner  layer  are  found  to  spread  rapidly,  so  that  during  the  expansion 
they  are  more  or  less  widely  separated  from  one  another.  But  they  continue  their 
expansion  and  multiplication  until  they  form  a  complete  inner  epithelial  layer. 

The  point  where  the  inner  mass  and  the  subzonal  layers  are  connected  with 
one  another  marks  the  site  of  the  future  embryonic  area. 

The  blastodermic  vesicle  grows  rapidly  in  size,  partly  by  the  multiplication  of 
its  cells,  partly  by  their  becoming  flattened  out  so  as  to  cover  a  larger  surface. 


THREE  SUCCESSIVE  STAGES. 

EC,  Outer  or  subzonal  layer,  z,  x,  Zona  pellucida. 
i.m,  Inner  mass  of  cells.  Ent,  Entoderm. — 
(After  W.  Heape.) 


THE  EMBRYONIC  SHIELD. 


47 


The  interior  of  the  vesicle  is  filled  with  fluid.  As  the  vesicle  grows  the  fluid  in- 
creases in  amount,  and  is  presumably  derived  by  the  ovum  from  the  walls  of  the 
uterus.  It  is  under  pressure  within  the  vesicle,  as  is  shown  by  the  manner  in  which 
it  spurts  out  if  the  vesicle  is  broken.  Nothing  exact  as  to  the  composition  of  this 
fluid  is  known,  though  we  may  suppose  it  to  resemble  more  or  less  the  serous  fluid 
of  the  adult  body.  The  size  and  form  of  the  vesicle  offer  characteristic  variations 
in  mammals.  It  starts  as  a  more  or  less  nearly  spherical  body.  In  the  rabbit  it 
assumes  an  oval  shape,  and  by  the  seventh  day  measures  about  4.0  mm.,  and  soon 
thereafter  becomes  attached  to  the  wall  of  the  uterus.  In  the  hedgehog,  the  guinea- 
pig,  and  the  mouse  the  ovum,  while  very  small  and  more  or  less  rounded  in  form, 
becomes  imbedded  in  uterine  tissue  and  develops  into  a  special  shape  in  adapta- 
tion to  its  new  situation.  In  the  ungulates  the  vesicle  grows  enormously,  becoming 
a  very  long  and  slender  sac.  Thus,  for  example,  in  the  sheep  it  may  measure 
on  the  fourteenth  day  not  less  than  50  cm.  in  length. 

Another  respect  in  which  the  blastodermic  vesicles  differ  greatly  from  one  an- 
other in  various  mammals  is  in  regard  to  the  early  development  of  the  subzonal 
layer,  or,  as  we  may  call  it,  the  ectoderm.  In  many  cases  the  entire  layer  under- 
goes a  precocious  development,  its  cells  multiply  very  rapidly,  so  that  the  layer 
becomes  several  cells  thick.  This  thickened  layer  is  known  as  the  trophoderm.  In 
other  placental  mammals  this  thickening  is  confined  to  a  limited  area  of  the  ecto- 
derm. For  further  description  see  Trophoderm,  page  114. 

The  Embryonic  Shield. 

Sooner  or  later  in  the  early  history  of  every  blastodermic   vesicle,   and  always 

as   the   first   indication   of  the  development  of  the  embryo   proper,   there  appears   a 

thickening  of  a  small  oval  area  of  the  outer  layer  in  the  region  of  the  inner  mass. 


FIG.  13. — TRANSVERSE  SECTION  THROUGH  THE  EMBRYONIC  SHIELD  OF  THE  BLASTODERMIC  VESICLE  OF  A  DOG 

OF  ELEVEN  OR  FIFTEEN  DAYS  (PREQISE  AGE  UNKNOWN).  , 

O.L,  Outer  layer.     Ent,  Entoderm.     X   2oo  diams. — (After  Bonnet.) 

This  thickening  is  known  as  the  embryonic  shield.  In  the  fresh  specimen  it  marks 
itself  by  the  greater  opacity  which  it  causes  in  the  walls  of  the  ovum  where  it  lies. 
In  those  cases  where  the  thickening  of  the  ectoderm  to  form  the  trophoderm  ex- 
tends over  the  entire  blastodermic  vesicle,  it  is  very  difficult  to  follow  the  early 
history  of  the  embryonic  shield.  In  other  cases,  however,  where  the  trophoderm 
occupies  a  special  restricted  area,  the  history  of  the  embryonic  shield  may  be  more 
readily  followed.  The  animals  in  which  it  has  hitherto  been  chiefly  studied  are 


48  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

the  rabbit,  dog,  cat,  and  sheep.  In  all  of  these  the  embryonic  shield  is  simply  a 
thickening  of  the  outer  layer  (Fig.  13).  The  embryonic  shield  is  at  first  small,  but 
it  rapidly  expands  and  assumes  a  rounded  or  oval  form.  There  next  appears,  in 
a  more  or  less  central  position  in  the  shield,  a  small,  darker  spot,  which  marks 
what  is  known  as  the  primitive  knot,  a  peculiarity  of  which  is  that  it  corresponds 
to  an  intimate  union  of  the  cells  of  the  inner  with  those  of  the  outer  layer  of  the 
blastodermic  vesicle.  (Compare  Fig.  126,  B,  page  171.)  Soon  a  linear  shadow  be- 
comes visible  extending  from  the  primitive  knot  toward  a  point  at  the  periphery 
of  the  embryonic  shield  (Fig.  14)  which  represents  the  embryonic  shield  of  a  dog 


Oo° 


Kn. 


O    O  o 


FIG.  14. — SURFACE  VIEW  OF  THE  EMBRYONIC  SHIELD  OF  THE  BLASTODERMIC  VESICLE  OF  A  DOG  OF  THIRTEEN 

TO  FIFTEEN  DAYS  (PRECISE  AGE  UNKNOWN). 

The  specimen  had  been  preserved  with  sublimate  and  stained  with  borax-carmin.    Sh,  Embryonic  shield.     Kn, 
Hensen's  knot,     p.s,  Primitive  streak.     X  100  diams. — (After  Bonnet.) 

at  about  two  weeks.  The  shadow,  p.s,  from  the  primitive  knot  is  termed  the 
primitive  streak,  and  it  very  soon  becomes  further  characterized  by  the  formation 
of  a  fine  groove  caused  by  a  depression  in  the  outer  layer  of  cells.  This  is  known 
as  the  primitive  groove,  and  has  been  observed  in  all  amniote  embryos.  Its  exact 
significance  has  never  been  satisfactorily  ascertained,  and  its  interpretation  is  still  a 
matter  of  scientific  discussion.  A  transverse  section  through  the  primitive  streak  of 
a  vesicle  of  a  common  European  mole  is  shown  in  figure  15.  At  about  the  time  the 
primitive  streak  appears  the  embryonic  shield  becomes  oval  in  form.  In  those 
animals,  such  as  the  carnivora  and  ungulates,  which  have  a  large  elongated  blasto- 
dermic vesicle,  we  find  that  the  long  axis  of  the  embryonic  shield  is  nearly  at 


GROWTH  OF  THE  EMBRYO  AND  SEPARATION  OF  THE  YOLK. 


49 


right  angles  to   the  long  axis   of  the  vesicle.     The  size  of  the  shield  is   about  the 
same  in  all  mammals  which  have  been  heretofore  studied. 

Growth  of  the  Embryo  and  Separation  of  the  Yolk. 

In    all    vertebrates    the    development    is    strictly    of    the    embryonic    type,    and 
accordingly  there  is  made  for  the  nutrition  of  the  embryo  some  special  provision, 


?r 


FIG.  15.— TRANSVERSE  SECTION  THROUGH  THE  PRIMITIVE  STREAK  OF  AN  EMBRYO  MOLE. 

EC,    Ectoderm.     En,    Entoderm.     mes,   Mesoderm.     p.gr,   Primitive  groove.     Pr,  Primflfve  streak. — (After  W. 

Heape.) 

which  in  most  cases  consists  of  a  stock  of  yolk  material;  but  in  the  placental 
mammals  the  provision  is  made  by  means  of  the  .placenta  for  the  transfer  of 
nutriment  directly  from  the  mother.  In  either  case  the  embryo  has  merely  to 
assimilate  the  food  already  more  or  less  prepared  for  it.  It  is  perhaps  owing  to 
these  provisions  that  the  growth  of  the  vertebrate  embryo  is  extremely  rapid.  In 
the  amniota  there  is  a  fundamental  distinction  between  the  embryo  proper  and  its 


bi 


FIG.  16. — DIAGRAMS  TO  ILLUSTRATE  THE  SEPARATION  OF  THE  EMBRYO  FROM  THE  YOLK. 
bl,  Blastopore.     h,  Head  of  embryo.     Ach,  Archenteron  or  entodermal  cavity,     ec,  Ectoderm. 

so-called  appendages — the  yolk-sac,  chorion,  amnion,  and  allantois.  The  append- 
ages are  all  finally  sacrificed  for  the  benefit  of  the  embryo,  and  in  mammals,  except 
for  a  portion  of  the  allantois  retained  in  the  body  as  the  anlage  of  the  bladder, 
the  four  appendages  are  ultimately  cast  off  altogether  and  take  no  part  in  the 
construction  of  the  child  after  birth.  We  note,  in  fact,  as  we  ascend  the  verte- 

4 


50 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


Op.L. 


Ao. 
Ph. 

Mk. 


brate'  series,  an  increasing  tendency  to  give  the  embryo  prominence  and  to  differ- 
entiate it  more  decisively  from  the  embryonic  appendages.  This  becomes  so 
marked  in  the  higher  vertebrates  that  we  speak  of  the  growth  of  the  embryo 
T0  epen  .  almost  as  a  separate  thing  from 

the  growth  of  the  appendages. 

The  embryo  is  developed  from 
the  axial  portion  of  the  embryonic 
shield,  the  position  of  which  is 
marked  by  the  primitive  streak 
(Fig.  14,  p.s).  In  the  territory 
around  the  embryo  are  developed 
the  first  blood-vessels,  hence  it  is 
termed  the  area  vasculosa  (see 
Per.cce.  Page  66).  About  the  time  that 
the  blood-vessels  begin  to  appear, 
the  separation  of  the  embryo  from 
the  shield  commences,  and  the 
extra-embryonic  portion  of  the 
shield  remains  as  part  of  the 
blastodermic  vesicle,  or  yolk-sac. 
This  separation  is  due  wholly  to 
the  growth  of  the  embryo.* 
The  process  is  illustrated  by  the 
diagrams  (Fig.  16),  in  which  for 
greater  clearness  the  blastodermic 
vesicle  is  represented  filled  with 
yolk,  as  it  is  in  the  Sauropsida. 
Soon  after  the  blood-vessels  ap- 
FIG.  17.— TRANSVERSE  SECTION  OF  AN  EMBRYO  CATFISH  pear,  the  head  of  the  embryo  has 

(AMIURUS);  SERIES  25,  SECTION  43.  grown    so    much    that  it  not  only 

Ao,  Aorta,     bas.g,  Basal  ganglion  of  mid-brain.     EC,  Ectoderm.        •  Hn          +fc  f  f     tVi 

epen,  Ependymal  layer  of  mid-brain,     it,  Cavity  of  mid-brain. 

L,    Lens.      Mk,    Meckel's    cartilage.      N.op,    Optic    nerve,      shield,   but    projects   forward    (Fig. 
Op.  L,  Optic  lobe.    Per.cce,  Pericardial  ccelom.     PA,  Pharynx.      16,  A,   K).       Later    the  caudal  end 
^Pigment  layer  of  the  eye     *,  Retina.    To,  Torus.    Trab,      becomes     free     in     the     same 
Trabecula   cranu.      x,    Undetermined    organ.       Yk,   Yolk. 

X4odiams.  (Fig.    1 6,    B,    C).     Cross  sections 

show  a   similar  expansion  of  the 

embryo  laterally  (compare  the  three  diagrams,  Figs.  29,  45,  A,  and  45,  B).  '  Hence, 
though  the  connection  between  the  embryo  and  the  blastodermic  vesicle  may  remain 

*  The  separation  of  the  embryo  from  the  rest  of  the  ovum  has  long  been  described  as  a  process  of  the  folding 
the  germ  layers  on  the  under  side  of  the  body.     The  traditional  perpetuation  of  this  erroneous  description 
s  regrettable,  for  the  separation  of  the  embryo  is  really  due  to  the  expansion  of  the  embryo,  and  in  no  sense  to  the 
constriction  of  its  connection  with  the  yolk. 


Ec.—\ 


ORIGIN  OF  THE  MESODERM.  51 

unchanged,  or  even  slightly  increase  in  dimension,  yet  the  growth  of  the  embryo 
causes  that  connection  to  appear  relatively  small.  A  connection  of  i  or  2  mm. 
equals  at  first  the  entire  length  of  the  embryo,  but  a  connection  of  4  or  5  mm. 
seems  small  when  the  embryo  is  100  or  200  mm.  long. 

The  relations  of  the  embryo  to  the  yolk  in  the  anamniota  are  illustrated  by 
the  accompanying  figure  17,  which  represents  a  transverse  section  through  a  young 
stage  of  the  catfish  (Amiurus).  The  section  passes  through  the  head  of  the 
embryo  and  shows  both  eyes  and  the  slender  optic  nerves,  N.op,  almost  symmet- 
rically cut  on  both  sides.  The  yolk,  Yk,  is  a  a  large  mass  heavily  laden  with  yolk- 
granules.  Between  the  tissues  of  the  embryo  proper  and  of  the  yolk-sac  there  is 
a  direct  continuity.  Not  only  can  the  ectoderm,  EC,  be  followed  around  from  the 
embryo  over  the  yolk-sac,  but  also  a  layer  of  mesoderm.  The  part  of  the  yolk-sac 
which  carries  the  yolk  grains  is,  as  above  stated,  a  modification  of  the  entoderm. 
There  is  no  amnion. 

Origin  of  the  Mesoderm. 

The  development  of  the  primitive  streak  and  groove  is  accompanied  by  the 
appearance  of  the  third  or  middle  germ-layer,  the  mesoderm  (Fig.  15,  mes).  As 
shown  in  the  section  there  figured,  the  three  germ-layers  are  fused  together  under- 
neath the  primitive  groove,  and  are  there  thicker  than  elsewhere.  As  we  pass 
laterally  from  the  groove,  the  ectoderm  and  mesoderm  both  become  thinner  and  are' 
distinctly  separated  from  one  another.  The  entoderm  consists  of  a  single  thin 
layer  of  cells  very  closely  connected  with  the  mesoderm.  The  mesoderm  occupies 
at  first  only  a  small  area  in  the  immediate  neighborhood  of  the  primitive  streak. 
It  grows  rapidly,  so  that  its  edge  extends  farther  and  farther  over  the  blastodermic 
vesicle.  The  mesoderm  is  to  be  regarded  as  the  product  of  the  entoderm.  Its 
exact  origin  in  mammals  has  not  yet  been  adequately  traced.  We  know,  however, 
that  in  birds,  reptiles,  and  elasmobranchs  the  cells  of  the  inner  layer  multiply 
rapidly,  so  that  the  inner  layer  becomes  more  than  one  cell  thick.  The  upper 
cells  soon  split  off  from  the  lower  and  thus  form  themselves  into  the  middle  germ- 
layer.  The  mesoderm  therefore  is  said  to  be  formed  by  delamination.  It  seems 
probable  that  in  mammals  the  process  is  the  same. 

It  may  be  mentioned  that,  according  to  Bonnet,  the  development  of  the  meso- 
derm in  the  sheep  is  not  quite  as  above  described.  It  can  be  first  distinguished 
at  the  stage  when  the  primitive  knot  has  appeared,  and  before  the  primitive  streak 
is  developed.  In  the  fresh  specimen  it  is  seen  as  a  slight  turbidity  of  the  vesicular 
walls  just  outside  the  edge  of  the  shield  (Fig.  18),  while  in  the  region  of  the  shield 
there  is  no  middle  layer  whatever.  By  the  time  the  primitive  streak  has  appeared 
in  the  sheep,  the  formation  of  the  mesoderm  has  extended  under  the  embryonic 
shield,  and  the  relations  between  the  germ-layers  then  become  essentially  as  above 
described. 

The  cells  of  the  mesoderm  are  at  first  quite  closely  packed,  but  as  the  layer 


52 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


grows  they  begin  to  move  apart,  though  remaining  connected  with  one  another  by 
protoplasmic  processes.  The  cells  separate  least  near  the  primitive  streak,  but  their 
separation  becomes  gradually  more  and  more  marked  toward  the  periphery  of  the 
layer,  as  shown  in  Fig.  19,  which  represents  a  part  of  the  peripheral  region  of  the 
mesoderm  of  a  blastodermic  vesicle  of  a  rabbit  of  seven  days. 

In  the  details  of  its  expansion  the  mesoderm  varies  greatly  in  different  mammals. 
In  some  forms  it  develops  very  early  and  rapidly  expands  over  the  entire  blasto- 


m  


FIG.  18.— CENTRAL  PORTION  OF  A  SHEEP'S  BLASTO-  FIG.  19.— BLASTODERMIC  VESICLE  OF  A  RABBIT  OF 

DERMIC  VESICLE  OF  TWELVE  TO  THIRTEEN  SEVEN  DAYS.    PORTION  OF  THE  MESODERM 

-°AYS-  OF  THE  AREA  OPACA. — (After  Kdlliker.) 

Sh,  Embryonic  shield,  kn,  Hensen's  knot,  mes, 
Shadow  caused  by  mesoderm  developing  around 
the  shield.  X  34  diams. — (After  Bonnet.) 

dermic  vesicle,  which  then  becomes  three-layered.  This  seems  to  be  the  method  of 
its  growth  in  man  and  other  primates.  In  other  cases,  as  in  the  dog  and  cat,  it 
grows  more  slowly,  but  ultimately  encloses  the  entire  entoderm.  In  the  rabbit, 
on  the  contrary,  it  never  expands  more  than  about  three  fifths  of  the  way  over  the 
blastodermic  vesicle,  one  part  of  which,  therefore— viz.,  that  opposite  the  embryo— 
never  has  any  mesoderm  whatever.  This,  however,  is  to  be  regarded  as  a  special 
modification,  since  we  must  consider  that  primitively  the  mesoderm  extended  over 
the  entire  vesicle. 

The  Primitive  Axis. 

The  next  stage  of  development  is  characterized  by  the  appearance  of  an  accu- 
mulation of  cells  which  extends  forward  from  the  primitive  knot  in  the  axial  line. 


THE  NOTOCHORDAL  CANAL.  53 

This  thickening  is  termed  the  primitive  axis.  German  writers  commonly  designate 
it  as  the  "head  process"  (Kopffortsatz) .  The  primitive  axis  may  be  easily  distin- 
guished in  transverse  sections  from  the  primitive  streak  by  the  fact  that  in  the  for- 
mer the  thickening  occurs  in  the  mesoderm  and  entoderm,  which  are  closely  united, 
and  it  is  separated  from  the  outer  layer;  whereas  in  the  latter  the  cells  of  the 
thickening  are  fused  with  both  the  entoderm  and  the  ectoderm  (compare  Fig.  126, 
A  and  C,  page  171). 

The  primitive  axis  corresponds  to  the  region  in  which  the  body  proper  of  the 
embryo  develops,  and  represents  the  beginning  of  embryonic  development  in  this 
restricted  sense.  It  grows  quite  rapidly  in  length  and  width,  and  as  it  grows  en- 
croaches more  and  more  upon  the  territory  of  the  primitive  streak,  which  is  grad- 
ually obliterated  by  merging  into  the  caudal  end  of  the  developing  embryo,  so  that 
it  can  no  longer  be  distinguished.  The  obliteration  of  the  primitive  streak  is  grad- 
ual, and  there  is  a  series  of  stages  easily  observed  in  amniota  in  -which  we  find  the 
embryonic  development  in  the  region  of  the  primitive  axis  more  or  less  advanced, 
while  part  of  the  primitive  streak  still  presents  to  us,  more  or  less  clearly,  its 
original  condition. 

The  Notochordal  Canal. 

In  regard  to  this  canal  our  knowledge  is  imperfect.  Any  account  of  it  which 
we  can  give  may  need  correction.  It  is  a  very  small  canal  which  runs  through  the 
center  of  the  primitive  axis.  It  ends  blindly  in  front,  but  opens  through  the  ecto- 
derm at  its  posterior  end,  at  a  point  corresponding  perhaps  exactly  to  the  position 
of  the  primitive  knot.  The  first  indication  of  the  formation  of  the  canal  is  an  al- 
teration in  the  form  of  the  cells  in  the  center  of  the  primitive  axis.  These  cells 
elongate  in  directions  at  right  angles  to  the  axis.  Their  nuclei  become  oval  and  are 
radially  placed.  The  change'  begins  posteriorly  and  progresses  forward.  The  radial 
cells  move  apart,  so  that  there  arises  a  longitudinal  canal.  It  may  happen  that  in 
mammals,  as  in  birds,  the  canal  is  not  actually  open  at  its  posterior  end.  If  that 
should  be  found  to  be  the  case  in  any  instance,  it  would  not  alter  our  interpreta- 
tion, for  we  should  then  consider  that  the  walls  had  simply  closed  togethe  r.  There 
are  many  instances  of  tubular  structures  being  temporarily  solid  in  embryonic  stages. 
Such  a  condition,  for  example,  has  been  observed  in  the  oesophagus  of  elasmo-^ 
branchs,  in  the  large  intestine  of  birds,  and  in  other  cases. 

The  opening  of  the  notochordal  canal  is  termed  the  blastopore,  and  is  suppose'd 
to  be  identical  with  the  blastopore  of  the  anamniota. 

After  the  notochordal  canal  is  formed  the  blastodermic  vesicle  has,  of  course, 
two  cavities:  first,  the  small  cavity  of  the  canal;  second,  the  large  main  cavity  of 
the  vesicle  which  is  surrounded  by  entoderm.  This  larger  space  is  designated  as 
the  yolk-cavity.  After  the  canal  has  acquired  a  not  inconsiderable  length  its  lower 
wall  develops  a  series  of  irregular  openings  (Fig.  20,  nch]  on  its  ventral  side,  by 
which  it  comes  into  communication  with  the  large  underlying  yolk-cavity.  These 


54 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


openings  grow  until  the  ventral  wall  of  the  notochordal  canal  is  entirely  lost.  We 
then  have  the  two  cavities  completely  fused,  making  a  single  cavity  bounded  by  a 
continuous  layer  of  cells,  the  majority  of  which  represents  the  lining  of  the  yolk- 
cavity,  but  the  small  minority  represents  the  cells  of  the  notochordal  canal.  The 
continuous  layer  of  cells  is  known  as  the  permanent  entoderm,  and  the  cavity  itself, 
which  is  of  double  origin,  is  termed  the  archenteron.  At  about  this  time,  probably 
sometimes  earlier,  sometimes  later,  according  to  the  species,  the  blastopore  becomes 
permanently  closed  and  the  entodermal  cavity  no  longer  has  an  opening  to  the 
exterior.  v 


i2Si  Atnn 


FIG.  20. — GERMINAL  AREA  OF  A  GUINEA-PIG  AT      FIG.    21. — LONGITUDINAL    SECTION    OF    THE    POSTERIOR 


THIRTEEN  DAYS  AND  TWENTY  HOURS,  SEEN 
FROM  THE  UNDER  (ENTODERMAL)  SIDE. 
o.  a,  Area  opaca.    a.p,  Area  pellucida.     nch,  Noto- 
chordal canal  with  several  irregular  openings 
through  the  entoderm. — (After  Lieberkilhn.) 


END  OF  A  SHEEP  EMBRYO  OF  SIXTEEN  DAYS. 
Amn,   Amnion.     a.m,   Anal   membrane   (or  plate),     pr.s, 
Primitive  streak.     En,  Entoderm.     Ach,  Archenteron, 
or   entodermal    cavity   of   the   embryo.     All,  Anlage 
of  allantois.     mes,  Mesoderm. — (After  R.  Bonnet.) 


In  a  number  of  vertebrates  it  has  been  demonstrated  that  the  blastopore  is 
soon  divided  into  two  parts:  one  anterior,  which  frequently  remains  open,  and 
gives  rise  to  the  neurenteric  canal,  and  one  posterior,  which  gives  rise  to  the  anal 
opening.  When  the  spinal  cord  (medullary  canal)  is  developed  it  extends  so  far  as 
to  include  the  neurenteric  canal  and  exclude  the  anus.  The  neurenteric  canal  is 
obliterated  during  early  embryonic  life,  but  so  long  as  it  remains  open  it  constitutes 
a  free  communication  between  the  archenteron  and  the  medullary  tube  (spinal 
cord).  The  anal  opening  is  early  closed  by  a  growth  of  the  surrounding  cells, 
which  produces  an  occluding  membrane  known  as  the  anal  plate  (Fig.  21,  a.m). 
The  plate  includes  a  layer  of  ectodermal  and  of  entodermal  cells,  but  apparently 
no  mesoderm.  It  persists  for  a  long  time  and  undergoes  a  considerable  growth, 
but  ultimately  it  is  perforated  to  form  the  permanent  anus. 

The  cells  on  the  dorsal  side  of  the  notochordal  canal  have  a  different  destina- 
tion, for  they  become  thickened  to  make  the  anlage  of  the  future  notochord.  It 
is  to  this  fact  that  the  canal  owes  its  name. 


THE  NOTOCHORD. 


55 


The  Notochord. 

The  notochord  (chorda  dorsalis)  is  a  rod  of  peculiar  tissue  constituting  the 
primitive  axial  skeleton  of  vertebrates.  It  begins  in  the  embryo  immediately 
behind  the  pituitary  body  and  extends  to  the  caudal  extremity.  It  occurs  as  a 
permanent  structure  in  all  vertebrates,  but  undergoes  much  modification  in  the 
amniota.  It  appears  very  early  in  the  course  of  development,  being  differentiated 
from  the  median  dorsal  wall  of  the  notochordal  canal,  beginning  at  a  time  when 
the  medullary  groove  (compare  page  68)  is  not  fully  marked  out  posteriorly,  and  is 
nowhere  closed.  The  notochordal  anlage  can  be  first  detected  as  an  axial  band 
of  cells,  which  at  first  is  not  well  marked  off  from  the  mesoderm  of  the  primitive 
axis.  The  anlage  is  thicker  than  the 
adjacent  entoderm  (Fig.  22,  nek).  The 
differentiation  of  the  notochordal  cells 
begins  usually  at  the  anterior  end  of 
the  canal  and  progresses  backward.  It 
appears  merely  as  a  specialized  part  of 
the  entoderm  of  the  archenteron,  but 
has  a  very  sharp  demarcation.  via 


FIG.  22. — TRANSVERSE  SECTION  OF  A  MOLE  EMBRYO 
(HEAPE'S  STAGE  H). 


segment.      CCR,    Coelom.      En,  Entoderm.      nch, 

Notochdrd.     ao,     Aorta,     -vt.a,  Vitelline    artery. 

Som,  Somatic  mesoderm.    Spl,  Splanchnic  meso- 
derm.— (After  W.  Heape.) 


The  notochordal  anlage  separates  off 
and    the    entoderm    proper   closes   across 

under     it,     SO     that    the    notochordal    band       am,  Amnion.     Md,  Medullary  groove.     My,  Primitive 

lies  between  the  entoderm  and  the  over- 
lying ectoderm  (floor  of  the  medullary 
groove  or  canal).  The  two  primitive  germ- 
layers  come  into  actual  contact  in  the 

median  line,  along  which,  therefore,  when  the  notochord  first  separates  from 
the  entoderm,  there  is  no  middle  germ-layer  present.  This  condition  exists 
in  the  chick  with  eight  segments  described  in  Chapter  V.  The  separation 
of  the  anlage  does  not  take  place  at  the  anterior  extremity  of  the  notochord 
until  somewhat  later,  so  for  a  considerable  period  the  cephalic  end  of  the  noto- 
chord remains  fused  with  the  entoderm.  The  separation  from  the  entoderm  is 
effected  in  mammals  by  the  entoderm  proper  shoving  itself  under  the  notochord 
toward  the  median  line.  When  the  cells  from  one  side  meet  those  of  the  other 
they  unite  with  them  and  form  a  continuous  sheet  of  entoderm  below  the  noto- 
chordal cells.  The  process  of  separation  may  be  followed  easily  in  the  develop- 
ment of  the  frog  and  toad. 

After  its  separation  the  notochord  is  a  narrow  band  of  cells,  which  starts 
anteriorly  from  the  entoderm  (the  future  lining  of  the  alimentary  tract),  running 
backward  to  the  blastopore.  So  long  as  the  blastopore  or  neurenteric  canal  is 
open  the  notochord  terminates  in  the  epithelium  lining  it.  For  a  certain  period 
the  notochord  continues  to  grow  tailward  by  accretion  of  cells  from  the  walls  of 
the  blastoporic  passage;  and  after  the  canal  is  permanently  obliterated,  the  noto- 


56 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


chord  may  still  continue  to  lengthen  by  acquisitions  at  its  caudal  end  of  additional 

cells   from  the  primitive  streak. 

After  it  is  once  formed  as  a  band  of  cells, 
the  notochord  passes  through  various  changes  of 
form,  but  ultimately  becomes  a  cylindrical  rod  with 
tapering  extremities.  It  attains"  considerable  size  in 
the  embryos  of  most  vertebrates,  but  in  those  of 
placental  mammals  it  is  always  small.  It  is  prob- 
able that  in  mammals  the  notochord,  when  first 
separated  from  the  entoderm,  is  a  broad,  flat  band, 
and  that  this  band  subsequently  draws  together, 
diminishing  its  transverse  and  increasing  its  vertical 
diameter  until  it  has  acquired  a  'rounded  form. 
Finally  its  outline  becomes  circular  in  cross-section. 
This  series  of  changes  begins  near  the  anterior  end 
of  the  notochord  and  progresses  both  forward  and 
backward. 

In  later  stages  the  mes/)derm  again  grows 
across  the  median  line  of  the  embryo,  completely 
surrounds  the  notochord,  and  forms  a  special 
sheath  about  it.  Still  later  the  mesoderm  forms  a 
broad  envelope  around  the  notochord,  which  we 
can  soon  recognize  as  the  anlage  of  the  chondro style, 
out  of  which  the  vertebral  column  and  part  of  the 
base  of  the  skull  are  to  be  differentiated.  Very 
soon  (Fig.  23)  the  chondrostylic  anlage  shows  a 
series  of  transverse  discs  of  denser  tissue,  the 
anlages  of  the  intervertebral  ligaments,  the  broader 
light  spaces  between  the  discs  being  the  anlages  of 
the  vertebrae.  In  mammals,  the  notochord  assumes 
an  undulating  course,  which  may  be  slightly  irregular 
at  first.  The  typical  arrangement  is  shown  in  the 
figure— the  dorsal  summit  of  each  flexure  is  in- 
tervertebral, the  ventral  hollow  of  each  flexure  is 
vertebral. 


FIG.33.-NOTOCHORDANDCHONDROSTYLE      ^     UltimatC     ***     °f     the     NotOChOrd. 

OF  A  SHEEP  EMBRYO  OF  14.6  MM.  As    the  vertebral  column  develops,  the  notochord 

RECONSTRUCTION    FROM     SAGITTAL    slowly  disappears  in  the  regions  of  the  vertebra  and 
SERIES  nog.  SECTIONS  IQO-IQ?  ,1 

even  the  space  occupied  by  it  is  obliterated  by  the 

growth  of  the  body  of  the  vertebra.     In   the   intervertebral   discs,   on  the  contrary, 
the   notochord   persists  to   form   the  nuclei  pulposi  of   the   adult.      Each    nucleus   is 


THE  ARCHENTERON. 


57 


enlarged,  first,  by  the  withdrawal  of  the  notochordal  cells  from  the  vertebrae  into  the 
adjacent  intervertebral  discs;  second,  by  the  growth  of  the  tissue.  The  cavities 
occupied  by  the  nuclei  have  distinct  boundaries  and  present  characteristic  forms  in 
different  mammals.  The  sheath  of  the  notochord  is  lost,  the  walls  of  the  cells  dis- 
appear, the  tissue  becomes  a  syncytium  (Fig.  24)  of  granular  appearance,  and 
breaks  up  into  multinucleated  reticular  masses, 
making  an  irregular  network  the  meshes  of  which 
are  filled  with  a  more  or  less  homogeneous  sub- 
stance resembling  mucin,  that  does  not,  however, 
agree  with  mucin  in  its  reactions.  Tissue  of  this 
character  may  be  easily  observed  in  human  embryos 
of  the  third  and  fourth  month.  It  has  been  not 
infrequently  stated  that  the  notochord  disappears  in 
mammals,  and  that  it  contributes  to  the  formation 
of  cartilage.  Both  statements  are  now  known  to 
be  erroneous.  Owing  to  the  persistence  of  the 
nucleus  pulposus,  the  vertebral  joint  differs  funda- 
mentally from  all  other  joints  in  the  body  of  the  adult. 


FIG.  24. — PIG  EMBRYO  OF  150  MM. 
Notochordal     syncytium    from     nucleus 
pulposus.     X  800  diams. — (After  L. 
W.  Williams.) 


The  Archenteron. 

The  archenteron  comprises  the  entire  cavity  bounded  by  the  entoderm.  At 
first  it  consists  chiefly  of  the  cavity  of  the  yolk-sac  (Fig.  25),  but  as  it  also  in- 
cludes the  embryonic  entodermal  tract  its  development  in  the  embryo  greatly  pre- 
dominates as  growth  continues.  As  the  head  of  the  embryo  protrudes,  the  archenteron 
forms  a  cephalic  prolongation,  known  as  the  fore-gut  (Figs.  25,  Vd,  and  132,  Vd),  which 
ends  blindly  in  front,  but  opens  behind  (caudad)  into  the  general  archenteric  space, 
its  opening  being  termed  the  fovea  cardiaca,  fo.  Later  as  the  caudal  region  becomes 
protuberant  the  archenteron  sends  into  it  a  similar  blind  prolongation,  known  as 
the  hind-gut  (Fig.  25,  H.g).  .As  the  embryo  grows — compare  the  section  on  growth, 
page  49 — the  connection  between  the  embryo  and  the  yolk-sac,  which  seems  so 
large  in  early  stages  (Fig.  25),  increases  very  little,  and  therefore  becomes  relatively 
smaller.  It  never  attains  more  than  3  or  4  mm.  The  embryo,  on  the  contrary, 
grows  enormously  (Fig.  34),  and  there  is  a  corresponding  enormous  lengthening  of 
the  fore-gut  and  hind-gut.  The  former  is  the  anlage  of  the  pharynx,  oesophagus, 
and  stomach.  The  latter  is  the  anlage  of  the  large  intestine  and  most  of 
the  ileum. 

During  embryonic  life  the  archenteron  is  divided  by  the  obliteration  of  the  con- 
nection between  the  yolk-sac  and  the  embryonic  entoderm.  For  a  time  the  ori- 
ginal point  of  connection  is  marked  by  a  small  pouch  (MeckeVs  diverticulum)  of  the 
ileum.  The  pouch  normally  disappears,  but  as  an  occasional  anomaly  (arrest  of 
development)  it  persists  in  the  adult. 


58 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


The  Oral  and  Anal  Plates. 

These  two  structures  resemble  one  another.  Each  occupies  a  small  area  and  is 
formed  by  the  intimate  union  of  the  entoderm  with  the  ectoderm.  When  the  union 
is  first  formed  the  two  layers  are  distinct,  but  they  soon  fuse,  so  that  no  boundary 
can  be  recognized  between  them.  Ultimately  both  plates  break  down,  their  cells 


Am. 


Cho. 


Bs. 


-     U.A. 


Yk.s. 


fo. 


FIG.  25.— WAX  RECONSTRUCTION  OF  DANDY'S  HUMAN  EMBRYO  WITH  SEVEN  SEGMENTS  BY  FREDERICK  T.  LEWIS. 

All,  Allantois.  Am,  Amnion.  Bs,  Body  stalk.  Cho,  Chorion.  fo,  Fovea  cardiaca.  H.g,  Hind-gut.  Ht,  Heart. 
Kn,  Hensen's  knot.  Som,  Somatopleure  enclosing  the  pericardial  cavity.  U.A,  Umbilical  artery.  U.V, 
Umbilical  vein.  Vd,  Fore-gut,  ve,  Blood-vessel.  Yk.s,  Wall  of  yolk-sac.  X  40  diams. 

disappearing,  and  they  are  replaced  by  openings,  that  of  the  oral  plate  forming  the 
opening  between  the  mouth-cavity  and  the  pharynx,  that  of  the  anal  plate  forming 
the  primitive  anal  opening.  The  anal  plate,  before  it  breaks  down,  makes  a  con- 
siderable growth,  forming  an  epithelial  mass  which  plays  an  important  part  in  the 
anatomical  modeling  of  the  region.  The  oral  plate  disappears  very  early;  the  anal 
plate  much  later. 


THE  DIGESTIVE  CANAL. 


59 


As  soon  as  the  head  of  the  embryo  has  grown  so  much  as  to  project  as  an  in- 
dependent part,  we  find  that  the  oral  plate  lies  on  the  under  surface  of  the  head,  a 
little  in  front  of  the  heart  (Fig.  26).  The  pro-amnion,  pro. am,  arises  from  the 
somatopleure  enclosing  the  heart,  ht,  so  that,  when  the  oral  plate  becomes  perforate, 
the  cavity  of  the  entoderm,  Ent,  will  communicate  directly  with  the  cavity  enclosed 
by  the  pro-amnion,  or,  in  other  words,  with  the  permanent  amniotic  cavity.  Figure 
72,  o.pl,  shows  the  oral  plate  in  a  little  later  stage,  shortly  after  which  the  plate 
ruptures. 

A  similar  anal  plate  at  the 
posterior  end  of  the  embryo  also 
lies  within  the  amnion  (Fig.  21). 
This  figure  is  taken  from  a  sheep 
embryo  in  a  very  early  stage,  so 
that  the  anal  plate  appears  to  lie 
on  the  dorsal  side.  By  the  curl- 
ing ventralward  or  the  bending 
over  of  the  tail  end  of  the  young 
embryo  the  anal  plate  is  gradually 
transferred  or  rolled  over  on  to 
the  ventral  side,  where  it  perma- 
nently remains,  For  the  relation 
of  the  anus  to  the  blastopore  see 
page  54. 


pro.anv 


FIG.  26. — LONGITUDINAL  SECTION  OF  THE  HEAD  END  OF  A 

MOLE  EMBRYO,  STAGE  H. 

EC,  Ectoderm.  En,  Entoderm.  Ent,  Anterior  end  of  archenteric 
cavity  with  the  oral  plate  on  the  cardiac  side,  fb,  Fore- 
brain.  ht,  Heart,  m.b,  Mid-brain.  Mes,  Mesoderm.  nch, 
Notochord.  pro.am,  Pro-amnion. — (After  W.  Heape.) 


The  Digestive  Canal. 

The  digestive  canal  proper  is  developed  by  the  growth  and  modifications  of  the 
fore-gut  and  hind-gut.  The  division  between  the  two  is  a  point  in  the  ileum  corre- 
sponding to  the  original  connection  with  the  yolk-sac,  marked  in  the  fetus  by 
Meckel's  diverticulum. 

The  fore-gut  forms  the  pharynx  (and  lungs),  the  oesophagus,  stomach,  duode- 
num, and  part  of  the  ileum.  It  also  produces,  as  appendages  to  the  canal,  the 
liver  and  pancreas. 

The  hind-gut  forms  most  of  the  ileum  and  the  entire  large  intestine,  together 
with  the  caecum  and  appendix. 

The  entoderm  persists  as  the  permanent  epithelial  lining,  and  produces  all  the 
glands  of  the  digestive  tract.  It  remains  a  thin  layer  throughout  life.  The  meso- 
derm  forms  the  greater  part  of  the  walls,  furnishing  the  connective  tissue,  the 
smooth  muscle  layers,  and  the  peritoneum,  which  last  consists  of  the  original  meso- 
thelium  and  a  thin  layer  of  chiefly  fibrillar  connective  tissue. 

The  general  course  of  the  development  is  shown  by  figure  27,  which  represents 
outlines  of  the  entodermal  canal  in  three  human  embryos,  uniformly  magnified 
twelve  diameters.  An  earlier  stage  is  shown  in  figure  25. 


60 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


The  fore-gut  in  the  4.2  mm.  embryo  has  lengthened.  It  communicates  freely 
with  the  oral  cavity  proper,  the  limit  of  which  is  indicated  by  the  hypophysis,  Hy, 
which  is  of  ectodermal  origin.  The  cephalic  portion  of  the  canal  has  undergone  a 


FIG.  27.-— OUTLINES  OF  THE  NOTOCHORD  AND  ENTODERMAL  CANAL  OF  THREE  HUMAN  EMBRYOS.  A,  4.2  MM. 

B,  7.0  MM.       C,   13. 8  MM. 

Al,  Allantois.  Ch,  Notochord.  Col,  Large  intestine.  E,  Caudal  intestine.  Ep,  Epiglottis.  Hy,  Hypophysis. 
in,  Small  intestine.  La,  Larynx.  Li,  Liver.  Li.d,  Hepatic  duct.  Lu,  Lung.  Md,  Mandible.  N,  Renal 
anlage.  «?,  (Esophagus.  P,  Dorsal  pancreas.  St,  Stomach.  T,  Tongue.  Th,  Thyroid  gland.  Ur, 
Ureter.  W,  Wolffian  duct.  Yk.s,  Yolk-sac.  1,2,3,  nrst>  second,  and  third  gill- pouches.  Xi2diams. — 
(After  W.  His.) 

great  widening  to  form  the  pharynx,  with  its  characteristic  gill-pouches  (Fig.  27,  A, 
i,  2,  3) — compare  below.  From  the  caudad  end  of  the  pharynx,  the  anlage,  Lu,  of 
the  trachea  and  lungs  has  appeared  on  the  ventral  side.  From  the  pulmonary  an- 


THE  DIGESTIVE  CANAL.  61 

lage,  Lu,  to  the  hepatic,  Li,  extends  a  short  tube  which  comprises  the  future 
oesophagus,  stomach,  and  part  of  the  duodenum.  The  liver,  Li,  which  arose  as 
an  outgrowth  of  the  entoderm  at  the  fovea  cardiaca,  has  enlarged  and  become 
distinctly  an  appendage.  Between  the  liver  and  the  yolk-sac,  Yk.s,  is  a  short 
broad  tube,  the  beginning  of  part  of  the  small  intestine.  In  the  7.0  mm.  embryo, 
the  fore-gut  is  much  longer,  and  the  differentiation  of  the  oesophagus,  oe,  stomach, 
St,  and  duodenum,  from  which  the  anlage  of  the  dorsal  pancreas,  P,  has  developed, 
is  established.  The  liver  is  connected  with  the  duodenum  only  by  the  narrow 
hepatic  duct,  Li.d,  between  which  and  the  yolk -stalk,  Yk.s,  there  is  a  consider- 
able stretch  of  small  intestine.  In  the  13.8  mm.  embryo,  the  relations  have  been 
greatly  altered  by  the  growth  and  migration  of  the  stomach  (Fig.  27,  C,  Si)  which 
has  descended  from  its  original  position  into  the  abdomen,  so  that  it  is  caudad  of 
the  diaphragm,  and  lies  asymmetrically  placed  on  the  left  side  of  the  embryo.  The 
stomach  also  turns  so  that  its  cesophageal  end  is  toward  the  left,  its  duodenal 
end  toward  the  right,  and  further  revolves  so  that  its  left  surface  faces  ventrally. 
In  the  13.8  mm.  embryo,  the  migration  and  revolution  of  the  stomach  has  not  , 
been  completed.  The  descent  of  the  stomach  involves  the  elongation  of  the  oeso- 
phagus (Fig.  27,  C,  oe)  and  the  twisting  of  the  duodenum. 

The  hind-gut  has  a  simpler  history.  In  the  4.2  mm.  embryo  it  has  elongated 
and  terminates  blindly  in  the  tail.  Its  caudal  end  is  somewhat  enlarged  to  form 
the  cloaca,  into  which  open  also  the  Wolffian  ducts  and  allantois  (Fig.  27,  A,  W 
and  Al).  Between  the  cloaca  and  the  yolk-sac,  Yk.s,  extends  the  cephalad  por- 
tion of  the  hind-gut,  nearly  uniform  in  diameter.  In  the  7.0  mm.  embryo  the 
conditions  are  similar,  but  the  intestinal  portion  has  lengthened  and  bent  ventral- 
ward.  The  insertion  of  the  yolk-stalk,  Yk.s,  marks  the  apex  of  the  primitive 
intestinal  loop.  In  the  13.8  mm.  embryo,  the  loop  has  greatly  lengthened  and 
projects  into  the  cavity  of  the  umbilical  cord  (extra-embryonic  ccelom),  and  a  blind 
pouch,  Coe,  has  appeared,  the  anlage  of  both  the  caecum  and  the  appendix.  It 
marks  the  boundary  between  the  large  and  small  intestines,  which  as  yet  differ 
very  little  in  diameter. 

For  some  time  a  portion  of  the  intestine  lies  in  the  umbilical  cord,  and  may 
form  several  coils  there,  but  gradually  it  is  withdrawn  so  as  to  lie  wholly  within 
the  abdomen  proper. 

The  pharynx  undergoes  many  modifications  in  form,  and  also  produces  an  im- 
portant series  of  accessory  organs,  including  the  thyroid  gland,  the  tonsils,  and  the 
thymus.  It  comprises  the  cephalic  portion  of  the  fore-gut  and  originally  overlies 
the  heart  (Figs.  25  and  132).  The  stretch  of  the  fore-gut,  which  extends  from  the 
pharynx  to  the  fovea  cardiaca,  remains  at  first  short  and  narrow,  most  of  the  fore- 
gut  being  absorbed  in  the  pharynx,  which  is  produced  by  the  expansion  of  the 
entodermal  tube  toward  both  sides  of  the  neck;  but  the  dorso- ventral  diameter 
remains  small.  The  expansion  is  greatest  a  short  distance  behind  the  mouth,  and 
thence  diminishes  gradually  toward  the  oesophagus,  so  that  the  pharynx  of  the  em- 


62 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


bryo  has  a  rhomboidal  form  which  is  complicated,  however,  by  certain  irregularities. 
These  are  due  to  the  formation  of  the  gill-pouches,  of  which  there  are  four  distinct 


car. I. 


Hy. 


car.in 


pairs  in  mammals.  Some  authorities  maintain  that  the  ancestors  of 
had  five  pairs,  and  that  the  pair  lost  was  situated  between  the  present  third  and 
fourth  pairs  (compare  the  remarks  on  "Zimmermann's  arch,"  page  101).  Each  pouch 
is  a  lateral  pocket  of  the  fore-gut,  having  a  tapering  form,  the  apex  of  which  comes 
into  contact  with  the  ectoderm.  At  the  point  of  contact,  entoderm  and  ectoderm 
fuse  to  constitute  a  closing  plate,  similar  to  the  oral  and  anal  plates.  In  aquatic 
vertebrates  the  closing  plates  are  lost,  and  each  gill-pouch  becomes  a  true  gill-cleft. 

The  positions  of  the  closing  plates 
soon  after  their  formation  are  marked 
by  an  external  depression,  the  ecto- 
dermal  gill-pouch  (Figs.  89  and  94). 
The  columns  of  tissue  in  front  of  the 
first  pouch,  behind  the  last,  and  be- 
tween the  first  and  second,  the  second 
and  third,  and  the  third  and  fourth 
are  known  as  the  five  branchial 
arches.  The  first  arch  is  called  the 
mandibular,  the  second  the  hyoid. 
In  each  branchial  arch  an  aortic  arch 
is  developed  (see  page  99).  In  mam- 
mals each  pair  of  pouches  has  a 
characteristic  form  in  the  embryo  and 
a  characteristic  differentiation.  In  the 
12.0  mm.  pig  the  first  pouch  (Fig.  28, 
I),  has  a  broad  base,  tapers  toward 

apex     rises 

the     second 


IV. 


Bu. 


FIG.  28.—  PIG  EMBRYO  OF  1  2  MM.    SERIES  518.    OUTLINE  the     ectoderm,     and      its 
OF  THE  PHARYNX  AS  SEEN  FROM  THE  DORSAL  SIDE.  j     ,u        j         ,       .  , 

FROMAWAXMODELBYA.R.KILGORE.  tOWard     the     dorsal     Slde 

I,  II,  in,  iv,    Gill-pouches.     2,  3,  4,  5,  Aortic  arches,  pouch,  II,  occupies  a  more  horizontal 

Ao,    Aorta.     Bu,  Bursa   pharyngis.     car.  in,  internal      plane   and   in   form   somewhat   resembles 

arte01!"'    ^J^f  -^n"  T  ^^^     the    first,    with    which    it    is    partially 

artery.       Hy,  Hypophysis.      Oe,   (Esophagus.    X  22 

diams.  merged.       The     third     pouch,  III,    is 

much    smaller   and    is  expanded  at  its 

end  by  a  prolongation  downward  and  inward;  the  prolongation  has  a  somewhat 
tubular  form  and  extends  far  toward  the  aortic  end  of  the  heart;  in  the  dorsal 
view  of  the  model  it  does  not  show.  The  fourth  pouch,  IV,  is  much  smaller  than 
the  others;  it  resembles  the  third  pouch  in  having  a  ventral  prolongation,  but  is 
quite  variable  in  form. 

The  entodermal  epithelium  of  the  second  to  fourth  pouches  exhibits  certain 
specializations.  One  type  is  illustrated  by  the  tonsil  and  thymus—  the  epithelium 
is  thickened,  assumes  a  reticular  structure,  and  its  meshes  are  invaded  by  leucocytes. 


THE  YOLK-SAC. 


63 


Som 


Coe 


Another  type  is  illustrated  by  the  epithelial  bodies,  which  are  small  masses  of  com- 
pact cells  resulting  from  a  local  epithelial  growth,  and  penetrated  by  blood-vessels 
(sinusoids) — this  type  includes  the  parathyroid,  nodulus  thymicus,  and  post- 
branchial  body. 

The  first  gill-pouch  becomes  the  Eustachian  tube,  the  blind  distal  end  being 
expanded  into  the  tympanum. 

The  second  pouch  is  partly  obliterated,  but  its  ventral  part  is  converted  into 
the  tonsil. 

The  third  pouch  forms  an  epithelial  body,  the  nodulus  thymicus  (Fig.  194,  Nod) 
and  its  ventral  caecal  prolongation  is  converted  into  the  thymus.  Its  epithelium 
is  said  also  to  produce  a  parathyroid. 

The  fourth  pouches  give  rise  to  a  pair  of  parathyroids  and  to  the  post-branchial 
bodies,  which  develop  from  the  ventral  prolongations  of  the  pouches. 

The  thyroid  gland  begins  as  a  median  evagination  of  the  entoderm  on  the 
ventral  side  of  the  pharynx.  It  starts  very  early  Spl 

(human  embryo  of  3  mm.).  The  blind  end  of  the 
evagination  becomes  first  bilobed,  then  branching 
—the  branches  are  the  anlages  of  the  adult  fol- 
licles. The  duct  of  the  gland  is  soon  obliterated, 
but  its  point  of  origin  is  often  permanently  marked 
by  the  foramen  caecum  at  the  back  of  the  tongue. 

The  Yolk-sac. 

General  Morphology. — The  yolk-sac  is  the  con- 
tainer of  the  nutritive  yolk  destined  to  be  assim- 
ilated by  the  embryo.  The  principal  factor  in 
its  morphological  constitution  is  the  entoderm, 
which,  after  the  differentiation  of  the  definitive 
germ-layers,  contains  nearly  all  of  the  yolk  mate- 
rial. In  the  primitive  vertebrates,  as  exemplified 
by  the  marsipobranchs,  ganoids,  dipnoi,  and  am- 
phibia, we  find  this  yolk  material  lodged  in  the  C<B>  Coeiom.  in,  Intestinal  cavity.  Som, 

walls    Of    the    primitive    digestive    tract.       It  is  Situated  Somatopleure.  Spl,  Splanchnopleure. 

chiefly  on  the  ventral  side  of  this  tract  and  extends  from  the  point  where  the  heart 
is  formed  toward  the  tail  of  the  embryo  to  the  point  where  the  allantois  is  formed. 
In  other  words,  it  is  situated  in  a  region  ^corresponding  to  the  territory  of  the 
future  abdominal  cavity.  In  the  primitive  types  just  referred  to,  the  yolk-bearing 
entoderm  becomes  divided  into  distinct  cells  which  form  a  large  mass.  The  con- 
dition may  be  understood  from  figure  44,  which  represents  a  transverse  section  of 
the  early  stage  of  an  axolotl  embryo.  The  cavity  of  the  entodermal  canal  (digest- 
ive tract)  is  small.  It  is  bounded  on  its  dorsal  side  by  a  single  layer  of  cells 
distinctly  epithelial  in  their  development,  and  on  the  ventral  side  by  a  great  mass 


FIG.  29.— DIAGRAMMATIC  SECTION  OF 
THE  YOLK  OF  A  HEN'S  EGG  AT  AN 
EARLY  STAGE  TO  SHOW  THE  RELATION 
OF  THE  PRIMITIVE  ENTODERMAL 
CAVITY,  Ach. 


04 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


of  rounded  cells  heavily  laden  with  yolk-granules,  and  containing  conspicuously 
large  nuclei.  These  large  nuclei  differ  by  their  size  and  minute  structure  very 
much  from  the  other  nuclei  in  the  embryo.  The  corresponding  nuclei  in  higher 
animals  are  sometimes  called  parablast  nuclei.  Outside  of  the  entoderm  comes  the 
second  portion  of  the  yolk-sac,  the  splanchnic  leaf  of  the  mesoderm.  If  we 
imagine  the  amount  of  yolk  to  be  gradually  increased,  so  that  it  would  appear 
more  distinct  from  the  embryo  proper,  we  should  then  apply  to  it  the  term  extra- 
embryonic.  The  yolk-sac  of  the  higher  forms  differs  from  that  of  the  lower  forms 
only  by  its  size,  as  is  illustrated  by  figure  29,  which  represents  a  diagrammatic 
transverse  section  of  an  early  stage  of  the  chick,  before  the  formation  of  the 
amnion  has  begun.  The  essential  relations  may  be  seen  by  comparing  figures  29, 


Ent 


FIG.  30. — WALL  OF  THE  YOLK-SAC  IN  THE  REGION  OF  THE  AREA  OPACA  OF  A  CHICK  OF  THE  SECOND  DAY. 
Mes,  Mesoderm.     V,V,  Blood-vessels,  containing  a  few  young  blood-cells.     Ent,  Entoderm.     c,  Four  distinctly 

shown  entodermal  cells. 

44i  and  45.  As  shown  in  the  section  (Fig.  29),  the  yolk-sac,  if  we  may  so  call  it, 
is  completely  enclosed  by  the  somatopleure  of  the  embryo,  and  in  the  amniote 
embryo  the  condition  is  the  same.  The  yolk-sac  is  surrounded  by  the  somato- 
pleure, which,  however,  in  the  amniota  we  call  extra-embryonic.  The  extra- 
embryonic  somatopleure  around  the  yolk-sac  is  called  in  birds  the  membrana  serosa, 
and  in  mammals  the  chorion. 

In  amniota  we  can  distinguish  in  the  entoderm  of  the  embryo,  or  yolk-sac, 
three  distinct  regions.  The  first  of  these  includes  the  whole  of  the  entoderm  of  the 
embryo  and  a  certain  territory  around  it.  In  this  region,  after  the  earliest  stages 
are  passed,  the  entoderm  is  found  to  be  a  very  thin  layer  and  to  contain  very  few 
yolk-granules,  and  such  few  as  it  contains  are  small.  This  portion  of  the  ento- 
derm, therefore,  seems  translucent,  an  appearance  which  can  easily  be  noted  with 
the  naked  eye,  and  which  has  led  to  the  name  area  pellucida,  which  has  long  been 
applied  to  this  region.  The  region  all  around  the  area  pellucida  appears  in  the 
fresh  specimen  darker,  and  this  is  called  the  area  opaca,  the  second  region.  The 
entoderm  in  this  part  consists  of  columnar  cells  (Fig.  30,  c,  and  Fig.  31).  In  the 
chick  the  cells  are  high  cylinder  cells  of  somewhat  irregular  shape,  containing  a 
loose  network  of  granular  protoplasm.  The  lower  ends  of  the  cells  are  rounded 


THE  YOLK-SAC.  65 

and  projecting,  and  have  a  well-marked  border  of  dense  protoplasm.  The  nuclei 
are  variable  in  size,  but  for  the  most  .part  large,  often  three  or  four  times  greater 
in  diameter  than  the  neighboring  mesodermic  nuclei.  They  usually  have  one, 
sometimes  two,  conspicuous  nucleoli.  The  nuclei  always  lie  at  the  upper  or  basal 
ends  of  the  cells,  chiefly  near  one  side  of  the  cell.  The  cells  contain  yolk-grains 
which  appear  to  be  undergoing  resorption.  Toward  the  area  pellucida  the  cells 
are  smaller,  the  network  of  protoplasm  closer,  and  the  yolk-grains  are  either  absent 
altogether  or,  if  present,  small  in  size  and  few  in  number.  The  transition  to  the 
thin  entoderm  of  the  area  pellucida  is  quite  abrupt.  In  the  opposite  direction  the 
area  opaca  passes  gradually,  by  changing  its  structure,  into  the  general  mass  of  the 
yolk,  or  area  vitellina,  the  third  of  the  regions  of  the  yolk-sac,  so  called  because  it 
contains  the  bulk  of  the  yolk  material.  The  transition  of  the  area  opaca  into  the 


FIG.  31. — WALL  OF  THE  YOLK-SAC  IN  THE  REGION  OF  THE  AREA  OPACA  OF  A  RABBIT  EMBRYO  OF  THIRTEEN  DAYS. 
V,  Blood-vessels  containing  young  red  blood-cells,  bl.     mes,  Mesoderm. 

area  vitellina  is  marked  by  a  considerable  accumulation  of  cells  which  are  arising 
from  the  yolk.  This  accumulation  of  cells  is  called  the  germinal  wall.  It  is  the 
connecting-link  between  the  epithelium  on  the  dorsal  side  of  the  entodermal  cavity 
and  the  yolk  or  area  vitellina,  which  forms  the  .ventral  boundary  of  the  cavity.  If 
we  follow  successively  the  stages,  we  find  that  the  area  pellucida  grows  at  the 
expense  of  the  area  opaca,  and  the  area  opaca  at  the  expense  of  the  area  vitellina. 
These  facts  are  to  be  interpreted  as  phases  in  the  process  of  the  assimilation  of 
the  nutritive  yolk.  The  thin  cells  of  the  area  pellucida  are  those  in  which  the 
absorption  of  the  yolk  has  been  completed.  The  larger  cells  of  the  opaca  are 
those  in  which  the  assimilation  is  going  on,  and  it  can  be  easily  seen  that  it  is 
most  advanced  in  those  cells  which  are  nearest  the  embryo  and  least  advanced  in 
those  cells  which  are  nearest  to  the  germinal  wall.  In  mammals  the  area  pellucida 
is  well  marked  and  resembles  that  of  birds.  The  area  opaca  has  well-defined 
cylinder  cells  (Fig.  31)  which  have  rounded  ends,  but  are  much  smaller  than  in 
birds  and  contain  very  little  yolk  material.  Cells  of  this  character  extend  over  also 
what  we  should  call  the  area  vitellina,  which  does  not  present  the  special  features 
which  it  has  in  birds,  for  the  reason  that  the  yolk  in  mammals  is  so  small  in 
amount  and  the  yolk-sac,  therefore,  is  hollow.  Later  on  the  cells  pass  through 
degenerative  changes,  which  need  to  be  more  exactly  studied.  In  man  the  degen- 


66 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


erative  change  in  the  cells  of  the  yolk-sac  takes  place  very  early.  The  mesoderm 
of  the  yolk-sac  is  at  first  a  thin  layer.  Very  early  there  appears  an  angioblast,  or 
the  anlage  of  the  first  blood-vessels  and  blood.  In  all  cases  in  which  the  process 
has  been  accurately  followed  the  angioblast  makes  its  first  appearance  in  the  region 
of  the  area  opaca,  where  it  forms  a  network  of  primitive  blood-vessels  close 
against  the  surface  of  the  yolk.  The  region  occupied  by  these  blood-vessels  is 
called  the  area  vasculosa.  Its  boundary  in  the  direction  away  from  the  embryo  is 
everywhere  well  defined.  Gradually  the  development  of  blood-vessels  progresses 
from  the  region  of  the  area  opaca  into  the  region  of  the  area  pellucida  and  extends 
into  the  body  of  the  embryo.  We  even  have  the  embryo  almost  completely  sur- 


raes 


FIG.  32. — SECTION  OF  THE  YOLK-SAC  OF  A  YOUNG 

HUMAN  EMBRYO. 
En,  Entoderm.     mes,  Mesoderm.    v,  Blood-vessels. 

—(After  Keibel.) 


FIG.    33. — HUMAN    EMBRYO,    2.15    MM.    LONG. 
(After  W.  His.) 


rounded  by  a  region  of  extra-embryonic  blood-vessels — the  definitive  area  vasculosa. 
Now,  it  will  be  remembered  that  the  area  opaca  is  the  territory  in  which  the 
entodermal  cells  are  actively  assimilating  the  yolk,  and  we  must  believe  that  the 
blood-vessels  which  are  thus  early  developed  in  close  contact  with  the  cells  of  this 
area  are  destined  to  take  up  food  material  digested  by  the  entodermal  cells  and 
carry  it  to  the  embryo.  Hence  we  interpret  the  early  development  of  the  extra- 
embryonic  vessels  as  due  to  physiological  necessities. 

The  mesoderm  at  first  forms  a  very  thin  layer  over  the  angioblast.  It  next 
thickens  by  the  multiplication  of  its  cells,  and  we  can  then  distinguish  in  it  both 
the  outer  mesothelium  and  the  inner  mesenchyma.  The  mesothelium  is  the  per- 
manent external  cover  of  the  yolk-sac.  The  mesenchyma  grows  in  between  the 
primitive  blood-vessels,  and  finally  penetrates,  at  least  in  part,  between  the  blood- 
vessels and  the  entoderm  of  the  yolk-sac,  a  condition  which  is  reached  very  early 
in  the  human  embryo  (Fig.  32).. 

The  human  yolk-sac  is  characterized  by  its  small  size  and  by  the  precocious 
expansion  of  the  area  vasculosa,  so  that  in  the  very  earliest  stage  known  to  us  by 


THE  ORIGIN  OF  THE  NERVOUS  SYSTEM. 


67 


observation  blood-vessels  are  found  over  the  entire  sac.  At  the  beginning  of  the 
third  week  the  diameter  of  the  yolk-sac  is  about  equal  to  the  length  of  the  embryo 
(Fig.  25).  By  the  end  of  the  third  week  the  sac  has  become  distinctly  pear- 
shaped,  its  narrower  pointed  end  being  that  by  which  it  is  connected  with  the 
intestinal  canal  of  the  embryo  (Figs.  33,  34).  The  sac  continues  growing,  up  to 
the  end  of  the  fourth  week,  after  which  it  enlarges  very  slightly,  if  at  all.  Its 
diameter  is  only  from  7  to  n  mm.  It  is  then  a  pear-shaped  vesicle  attached  by 
a  long  stalk  to  the  intestine,  the  stalk  having  been  formed  by  the  lengthening  of 
the  neck  of  the  yolk-sac.  The  cavity  of  the  stalk  early  becomes  obliterated  and 
the«entoderm  in  the  stalk  disappears  altogether. 


FIG.  34. — HUMAN  EMBRYO  OF  2.6  MM.— (.4/ter  W.  His.) 

The  Origin  of  the  Nervous  System. 

It  will  be  remembered  that  the  ectoderm  of  the  embryonic  shield  has  at  first 
a  considerable  thickness,  for  it  consists  of  cuboidal  or  low  cylindrical  epithelial 
cells.  The  stage  which  follows  next  after  the  appearance  of  the  primitive  axis  is 
characterized  by  the  gradual  thinning  out  of  the  ectoderm  over  the  peripheral  por- 
tions of  the  shield,  while  in  the  neighborhood  of  the  axial  line  the  full  thickness  of 
the  outer  germ-layer  is  not  only  retained,  but  is  actually  increased.  For  a  time 
there  is  a  gradual  passage  between  thicker  and  thinner  parts,  but  as  development 
progresses  the  demarcation  rapidly  becomes  sharper.  By  these  steps  the  differentia- 
tion of  the  anlage  of  the  central  nervous  system  is  accomplished.  The  thicker  cen- 
tral portion  of  the  ectoderm  constitutes  the  medullary  plate,  which  begins  to  appear 
shortly  after  the  formation  of  the  primitive  streak..  It  extends  over  the  primitive 
axis,  the  primitive  knot,  and  the  anterior  end  of  the  primitive  streak  (Fig.  35,  Md), 
and  also  extends  some  distance  to  the  right  and  left  of  the  axial  line.  It  is  rounded 


68 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


A. p.       A.o. 


md.F 


in  front,  also  behind,  where,  however,  it  gradually  fades  out.  At  the  same  time 
that  the  medullary  plate  is  being  thus  differentiated,  the  central  portionbe  comes  de- 
pressed, making  the  conspicuous  furrow,  md.F,  which  begins  just  in  front  of  the  primi- 
tive knot  and  extends  nearly  to  the  anterior  edge  of  the  medullary  plate.  This 
axial  depression  is  known  as  the  dorsal  furrow.  Its  appearance  is  shown  in  cross- 
section  as  illustrated  by  figure  36,  /.  The  furrow  is  narrow  and  deep.  Its  upper 
edge  is  rounded  or  curving.  By  the  formation  of  the  furrow  the  ectoderm  of  the 

medullary  plate  is  brought  into  actual  con- 
tact with  the  anlage  of  the  notochord  (Fig. 
36,  ch),  so  that  the  mesoderm  can  be  no 
longer  in  the  median  line  and  is  conse- 
quently divided  into  right  and  left  parts,  as 
above  mentioned  in  describing  the  formation 
of  the  notochord.  As  the  blastopore  lies  at 
or  near  the  primitive  knot,  it  becomes 
partly  included  in  the  medullary  plate.  It 
may  remain  open  while  the  medullary  plate 
is  being  transformed  into  the  nervous  system,' 
and  in  that  case  may  establish  a  connection 
between  the  cavity  of  the  central  nervous 
system  and  that  of  the  entoderm.  Such  a 
communication  is  termed  a  neurenteric  canal. 
Figure  79  represents  a  wax  model  recon- 
structed from  the  sections  of  a  human  em- 
bryo in  the  stage  of  the  medullary  plate. 


Md 


Kn 


pr.s 


FIG.    35. — SURFACE    VIEW    OF    THE    EMBRYONIC 


SHIELD  OF  A  DOG  EMBRYO,  WITH  MEDULLARY      It:    snOWS    clearly   the   form   of   the   plate,    the 


PLATE. 


deep     dorsal    groove,    the    opening    of    the 
A.o,  Area  opaca.    A.p,  Area  pellucida.    Kn,  Hen-     neurenteric  canal,  and   the  remnants  of  the 

sen's    knot.     Md,     Medullary     plate.     md.F,  ...  . 

Medullary  furrow,    pr.s,  Primitive  streak,    x    .Primitlve  groove  behind  the  canal.      As  the 
15  diams.  development  progresses  the   medullary  plate 

extends    farther    backward    and    encroaches 
upon  the  territory  of  the  primitive  streak  until  this  latter  is  obliterated. 

The  Medullary  Groove.— Almost  or  quite  as  soon  as  the  medullary  plate  is 
formed  its  lateral  portions  begin  to  arise  on  each  side,  so  that  the  two  halves  of 
the  plate  together  form  a  broad  open  trough  known  as  the  medullary  groove, 
into  which,  of  course,  the  dorsal  groove  is  merged,  so  that  it  no  longer  can  be 
recognized  (compare  Figs.  22  and  147).  While  the  groove  is  being  formed 
the  medullary  plate  increases  considerably  in  thickness.  The  nuclei  multiply 
rapidly  and  lie  irregularly  scattered  at  various  heights.  The  ectoderm  alongside  the 
medullary  plate  or  groove  thins  out  still  further.  The  development  is  most  rapid 
at  a  point  corresponding  to  the  posterior  region  of  the  future  head.  The  farther 
from  this  point  we  go,  the  less  advanced  do  we  find  the  formation  of  the  groove. 


THE  STRUCTURE  OF  THE  MEDULLARY  CANAL. 


69 


so  that  at  a  certain  stage  there  is  a  well-marked  medullary  groove  in  the  cephalic 
region,  the  medullary  plate  behind  that,  and  the  primitive  streak  at  the  hind  end 
of  the  embryo.  But  when  the  streak  has  disappeared,  the  medullary  groove  is 
found  to  extend  the  entire  length  of  the  embryo.  Owing  to  this  peculiarity,  it  is 
possible  in  a  single  embryo  to  follow  all  the  principal  stages  of  the  formation  of 
the  medullary  groove  by  the  examination  of  a  series  of  transverse  sections.  Such 
a  stage  is  found  in  the  rabbit  at  nine  days, .or  in  the  chick  at  from  thirty  to  forty 
hours  of  normal  incubation  (Figs.  129,  130,  and  147). 

The  Medullary  Canal. — The  medullary 
groove  gradually  deepens,  its  sides  rising 
higher  and  higher  and  arching  more  and 
more  toward  one  another  until  the  edges 
meet  and  coalesce,  thus  changing  the  groove 
into  a  tube — the  medullary  canal  (Figs.  37 
and  38,  Md}.  The  closure  of  the  groove 
occurs  in  the  cervical  region  first,  and 
spreads  from  there  in  both  directions.  As 
the  closure  progresses  forward  it  completes 
the  canal  in  the  region  of  the  head.  It 
occurs  in  such  a  manner  that  there  is  a 
very  small  opening,  which  is  the  last  point 
to  close.  This  opening  seems  to  be  a  fixed  /,  Dorsal  furrow, 
point,  occupying  always  the  same  relative 
position  in  all  vertebrates.  It  is  called  the 
anterior  neuropore.  At  this  time  the  caudal 
end  of  the  medullary  groove  may  be  still 
open  widely,  forming  the  so-called  rhom- 

boidal  sinus,  compare  figures  129  and  130,  and  it  is  the  last  portion  to  close.  Of 
the  entire  length  of  the  primitive  canal,  about  one  half  is  the  anlage  of  the  brain, 
while  the  other  half  is  the  anlage  of  the  spinal  cord.  In  the  subsequent  develop- 
ment of  the  brain  the  transverse  expansion  of  the  canal  is  most  conspicuous,  while 
in  the  development  of  the  spinal  cord  the  elongation  of  the  canal  predominates. 
The  dilatation  of  the  brain  begins  very  early. 

The  medullary  canal  produces  the  entire  central  nervous  system.  Some  of  the 
cells  from  its  walls  migrate  out  of  the  wall  itself  on  either  side.  These  cells  pro- 
duce the  ganglia. 

The  Structure  of  the  Medullary  Canal. 

When  the  medullary  canal  is  first  formed,  it  tends  to  present  a  rounded  out- 
line in  transverse  section.  But  its  lateral  walls  being  thicker  than  the  wall  on  the 
dorsal  and  ventral  sides  of  the  canal,  the  internal  cavity  appears  somewhat  flattened 
(Fig.  37).  On  its  ventral  side  it  lies  against  the  notochord.  On  its  dorsal  surface 


FIG.  36. — CROSS-SECTION  OF  A  HUMAN  EMBRYO  OF 

I  .54  MM. 

,  Ectoderm,  ct,  Somatic  meso- 
derm.  p,  Beginning  of  the  embryonic  ccelom. 
g,  Junction  of  the  extra-embryonic  somatic  and 
splanchnic  mesoderm.  df,  Splanchnic  meso- 
derm.  en,  Entoderm.  me,  Mesoderm.  ch, 
Notochord. — (After  Count  Spec.) 


70 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


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THE  STRUCTURE  OF  THE  MEDULLARY  CANAL. 


71 


it  is  in  contact  with  the  overlying  ectoderm,  from  which  it  has,  however,  completely 
separated,  and  it  causes  the  overlying  ectoderm  to  rise  up  somewhat.  Its  sides  are 
in  contact  with  the  mesoderm,  which  is  there  developing  into  the  primitive  seg- 
ments, page  84.  The  nuclei  in  the  wall  of  the  canal  are  very  numerous,  oval  in 
form,  and  usually  with  a  single  nucleolus.  The  nuclei  are  placed  in  the  radial 
lines.  For  some  time  after  the  canal  has  become  closed  the  nuclei  multiply  very 
rapidly  by  indirect  division,  but  all  of  the  mitotic  figures  are  found  close  to  the 
inner  surface  of  the  canal,  which  surface,  it  will  be  remembered,  corresponds  to 
the  original  outer  surface  of  the  ectoderm. 


Md       Seg 


Cho, 


FIG.  38. — TRANSVERSE  SECTION  OF  A  RABBIT  EMBRYO  OF  EIGHT  DAYS  AND  Two  HOURS. 

Md,   Medullary   canal.    Seg,   Primitive   segments.     Cho,   Chorion.     Am,   Amnion.    Som,   Somatopleure.     C<e, 
Coelom.    Spl,  Splanchnopleure.     Ent,  Entoderm.     Ch,  Notochord.     Ao,  Aorta. 

The  differentiation  of  the  brain  and  spinal  cord  is  indicated  even  during  the 
stage  of  the  medullary  groove.  The  extreme  anterior  end  of  the  groove  is  found 
to  widen  out  so  as  to  produce  a  pair  of  lateral  expansions.  As  development  pro- 
gresses and  the  canal  closes,  these  expansions  become  more  marked  and  are  them- 
selves, of  course,  also  closed  over,  so  that  when  the  canal  is  completed  they  appear 
as  lateral  diverticula  or  evaginations  of  the  tube,  which  are  known  as  the  primary 
optic  vesicles  (Figs.  129  and  130).  While  the  vesicles  are  developing  the  medullary 
tube  expands  in  diameter  throughout  its  cranial  or  anterior  half  without  any  notice- 
able change  in  the  general  histological  structure  of  its  walls.  Very  soon  the  expansion 
becomes  unequal,  and  the  inequalities  are  such  that  they  produce  three  dilatations, 
which  are  known  as  the  three  primary  cerebral  vesicles  (Fig.  131).  The  first  vesicle 
is  in  the  region  of  the  optic  outgrowth,  the  second  is  just  behind  this,  and  the 
third  is  as  long  as  the  first  and  second  combined  and  merges  into  the  spinal  cord. 
At  the  time  these  vesicles  become  recognizable  they  occupy  about  half  the  entire 
length  of  the  rrfedullary  tube.  Between  the  first  and  second  vesicles  there  is  a  Con- 


72 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


striction,   and  one  also  between  the  second  and  the  third.     The  three  vesicles  are 
the  anlages,  respectively,  of  the  fore-brain,  mid-brain,  and  hind-brain. 

In  the  region  of  the  spinal  cord  the  medullary  tube  soon  becomes  somewhat 
flattened  from  side  to  side,  and  therefore  acquires  a  characteristic  oval  configura- 
tion as  seen  in  cross-section  (Fig.  38).  We  can  now  recognize  in  the  cross-sections 
four  regions:  first,  the  two  thick  sides;  second,  in  the  median  dorsal  line  the  thin 
portion  which  we  call  the  deck-plate,  and  in  the  median  ventral  line  the  thin  por- 
tion which  we  call  the  floor-plate.  Later  on,  each  lateral  portion  becomes  sub- 
divided into  two  longitudinal  bands,  known  as  the  zones  of  His,  and  distinguished 
from  one  another  as  the  dorsal  and  ventral  zones  (Fig.  116,  D.Z,  V.Z}.  After  this 
stage  there  are  six  longitudinal  zones  in  the  embryonic  cord.  These  are,  first,  the 
deck-plate;  second  and  third,  the  dorsal  zones  of  His;  fourth  and  fifth,  the  ventral 
zones  of  His;  and  sixth,  the  floor-plate.  These  six  zones  also  appear  in  the  region 
of  the  brain,  where,  however,  .they  undergo  characteristic  modifications.  The  zones 

of  His  dominate  the  entire  morphology 
of  the  central  nervous  system,  because 
a^  t^ie  sensory  nerves  enter  the  central 
nervous  system  at  the  lower  edge  of  the 
dorsal  zone  and  primarily  ramify  in  the 
dorsal  zone,  and,  further,  because  all 
efferent  nerve-fibers  arise  in  the  ventral 
zone  and  pass  out  to  the  body  at  certain 
points  on  the  surface  of  the  ventral  zone. 

Origin  of  Nerves. 

The  essential  constituents  of  a  nerve 
are   the   neuraxons,  each   neuraxon  being 

FIG.  39.— TRANSVERSE  SECTION  OF  THE  DORSAL  CORD      the   prolongation   of  a  nerve   cell.       In  ad- 
AND  GANGLION  OF  A  CHICK  OF  NINE  DAYS.  dition,     the    neuraxons    are    usually    sur- 

V.r,  Anterior  root.    Dr,  Posterior  root.    Gl,  Ganglion  of      roimded      by      medullary      sheaths.         The 
dorsal  root,     col,  Collateral,  with  its  branches.    N, 

Medullary  neuroblasts  with  dendrites  and  axis-  y°Ung  Cells  from  whlch  the  neuraxons 
cylinder.  (The  drawing  is  from  a  Golgi  prepara-  grow  Out  are  called  neuroblasts.  The 
tion.  No  sheath  or  neuroglia  cells  are  repre-  olfactory  nCrve  has  a  Special  historv 
sented.)— (After  Cajal.) 

(compare     page     76).        All     the    other 

nerves    are    produced    by    neuroblasts,    which    are    developed    from    the    cells  of  the 
medullary  tube. 

Shortly  after  the  medullary  groove  has  closed  to  form  a  tube,  a  number  of 
cells  migrate  from  the  medullary  wall,  just  at  the  junction  of  the  dorsal  zone  and 
the  deck-plate,  and  in  sufficient  numbers  to  form  a  longitudinal  band,  which  is 
known  as  the  ganglionic  crest,  which  soon,  however,  breaks  up  into  separate  masses, 
the  ganglia,  which  develop  symmetrically.  Some  of  the  cells  of  each  ganglion 
become  neuroblasts  (Fig.  39,  Gl},  each  of  which  forms  a  centripetal  process  which 


THE  SPINAL  CORD  AND  BRAIN.  73 

enters  the  dorsal  zone  of  the  medullary  tube,  and  a  centrifugal  process  which  passes 
downward  to  join  the  ventral  root.  Other  ganglionic  cells  are  converted  into  the 
medullary  sheaths  of  the  nerve-fibers,  and  still  others  migrate  along  the  nerve- 
libers  and  give  rise  to  the  peripheral  or  sympathetic  ganglia. 

The  cells  which  are  retained  permanently  in  the  medullary  wall  differentiate 
themselves  into  two  classes:  first,  the  so-called  spongioblasts,  which  become  the  neu- 
roglia  of  the  adult,  and  second,  the  neuroblasts  (Fig.  39,  N}.  Each  medullary  neuro- 
blast  produces  a  single  neuraxon  and  several  dendrites,  although  a  small  number 
in  certain  positions  remain  always  without  dendrites.  Most  medullary  neuraxons 
are  distributed  within  the  spinal  cord  or  brain,  but  some  of  those,  developed  from 
cells  in  the  ventral  zone  only,  leave  the  medullary  wall  to  produce  the  ventral  (Fig. 
39,  V.r]  and  lateral  nerve-roots.  Ventral  roots  constitute  the  third,  sixth,  and 
twelfth  cephalic  nerves,  and  enter  into  the  composition  of  all  the  spinal  nerves. 
Lateral  roots  form  part  of  the  fifth,  seventh,  ninth,  and  tenth  nerves,  and  the 
whole  of  the  eleventh  cephalic  nerve,  but  probably  take  no  part  in  the  formation  of 
any  true  spinal  nerve. 

The  gangiionic  crest  of  the  chick  is  figured  and  described  in  Chapter  V.  The 
ganglia,  roots,  and  nerves  of  the  pig  embryo  are  figured  and  described  in  Chap- 
ter VI. 

* 

The  Spinal  Cord  and  Brain. 

The  medullary  tube,  after  giving  off  the  cells  which  form  the  neural  crest, 
becomes  the  definite  anlage  of  the  spinal  cord  and  brain.  The  differentiation  of 
the  brain  begins  very  early,  and  is  marked  by  an  enlargement  of  the  medullary 
tube  in  the  region  of  the  future  head  (compare  Fig.  131).  As  seen  there,  the 
brain  takes  up  nearly  half  the  entire  length  of  the  medullary  tube.  The  widening 
of  the  anterior  portion  of  the  medullary  canal  is  not  uniform,  but  tripartite.  The 
brain  is  divided  by  two  narrower  parts  into  three  wider  divisions,  which  are 
termed  the  primary  cerebral  vesicles,  and  are  named  in  their  order— fore-brain,  mid- 
brain,  and  hind-brain.  The  fore-brain  widens  very  rapidly  so  ,as  to  form  two  lateral 
projections,  the  optic  vesicles  (Fig.  131,  op.V}.  The  division  between  the  mid-brain 
and  the  hind-brain  is  at  first  indistinct,  but  soon  becomes  sharply  marked  off.  The 
hind-brain  then  appears  about  as  long  as  the  other  two  vesicles  combined,  and 
tapers  down  toward  the  spinal  cord,  into  which  it  merges  without  demarcation. 

The  walls  of  the  medullary  tube  acquire  throughout  certain  fundamental  char- 
acteristics, which  are  best  studied  in  transverse  sections.  The  side  walls  become 
thickened,  but  the  median,  ventral,  and  median  dorsal  portions  remain  thin  (Figs. 
157,  158,  Sp.c):  The  upper  thin  part  is  called  the  deck-plate;  the  lower  thin  part, 
the  floor-plate.  Soon  each  lateral  wall  is  divided  into  two  longitudinal  bands, 
designated,  respectively,  the  dorsal  and  the  ventral  zone  (Fig.  116,  D.Z,  V.Z}.  In 
young  embryos,  as  shown  in  the  figure  just  cited,  the  two  zones  are.  separated 
from  one  another  by  an  internal  notch.  Their  morphological  characteristics  depend 


74  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

upon  their  relations  to  the  nerve-roots.  In  the  ventral  zone  are  lodged  all  the 
neuroblasts  which  produce  efferent  nerve-fibers,  which  fibers  constitute  the  ventral 
nerve-roots  in  the  region  of  the  spinal  cord  (Fig.  39),  and  in  the  region  of  the 
brain  and  the  medulla  oblongata  produce  both  lateral  and  ventral  roots.  Neuro- 
blasts are  also  differentiated  in  the  dorsal  zone,  but  the  neuraxons  which  they  send 
out  are  confined  in  their  distribution  to  the  central  nervous'  system  itself,  and  never 
share  in  the  formation  of  nerve-roots.  The  dorsal  zone  is  further  characterized  by 
the  fact  that  the  afferent  or  sensory  fibers  from  the  ganglia  enter  its  lower  edge 
and  have  their  first  distribution  within  the  dorsal  zone.  The  stratification  of  the 
thickened  portions  of  the  medullary  tube  begins  early.  It  is  initiated  by  the 
appearance  of  a  thin  superficial  layer,  the  ectoglia  (Fig.  «fc2i,  Ec.gl),  which  contains 
no  nuclei.  The  thick  nucleated  portion  of  the  medullary  wall  changes  in  appear- 
ance as  the  neuroblasts  are  differentiated.  This  occurs  in  such  a  way  that  there 
is  always  a  layer  of  relatively  undifferentiated  cells  next  the  cavity  of  the  tube. 
This  is  known  as  the  ependyma  layer  (Fig.  121,  Eperi).  It  persists  throughout  life, 
remaining  thin,  and  never  containing  nerve-cells.  The  layer  between  the  ependyma 
and  the  ectoglia  is  called  the  gray  layer,  or  cinerea  (tin).  This  layer  grows  very 
rapidly,  and  the  enlargement  of  the  spinal  cord  and  brain  depends  chiefly  upon  the 
expansion  of  the  cinerea. 

The  three  cerebral  vesicles  pass  through  numerous  modifications  in  their  form 
and  cellular  structure,  yet  their  primary  morphological  characters  are  never  obliter- 
ated. The  brain  always  has  a  central  cavity,  the  boundary  walls  of  which  con- 
stitute the  organ.  The  fore-brain  produces  from  its  dorsal  zone  two  lateral  hollow 
outgrowths,  the  cerebral  hemispheres,  the  cavities  of  which  constitute  the  lateral 
ventricles  of  the  adult.  The  median  cavity  of  the  fore-brain  is  the  third  ventricle; 
the  communication  between  the  third  ventricle  and  the  lateral  ventricle  is  the 
foramen  of  Monro.  The  cavity  of  the  mid-brain  is  always  small,  and  its  walls  are 
greatly  thickened;  it  is  termed  the  iter  in  the  adult.  The  cavity  of  the  hind-brain 
is  much  enlarged  and  becomes  the  fourth  ventricle.  The  part  of  the  fore-brain 
from  which  the  hemispheres  arise  lies  farthest  cephalad,  and  is  termed  the  telen- 
cephalon.  It  is  marked  off  on  the  dorsal  side  by  a  transverse  fold,  the  velum 
transversum,  which  projects  inward.  The  part  of  the  fore-brain  caudad  from  the 
velum  is  called  the  diencephalon.  In  the  region  of  the  fourth  ventricle  we  can 
observe  that  the  cerebellum  and  the  pons  arise  from  the  region  adjoining  the  mid- 
brain.  This  region  is  called  the  metencephalon,  and  the  part  caudad  from  it — out 
of  which  the  medulla  oblongata  is  differentiated — is  called  the  myelencephalon.  The 
narrow  connection  between  the  mid-brain  and  the  hind-brain  is  termed  the  isthmus. 
The  following  table  indicates  the  relations  of  these  embryonic  divisions  to  the  adult 
parts.  In  Chapter  VI  the  essential  facts  of  brain  development  are  illustrated  and 
described. 


THE  SPINAL  CORD  AND  BRAIN. 


75 


TABLE  OF  BRAIN  DEVELOPMENT. 


Telencephalon 


i.  Fore-brain 

(Lateral  and  third  ventricles) 
Kl 


2.  Mid-brain 

(Iter) 


Diencephalon 


Mesencephalon 


Optic  vesicles. 
Hemispheres. 
Olfactory  bulb. 
Corpus  striatum. 
Lamina  terminalis. 
Infundibular  gland. 

Epiphysis. 
Thalamus. 
Tuber  cinereum. 
Pars  mammilaris. 

Corpora  quadrigemina. 
Cerebral  peduncles. 


3.  Hind-brain    

(Fourth  ventricle) 


Isthmus. 

Cerebellum. 

Pons. 


Metencephalon 

Myelencephalon Medulla  oblongata. 


R.d. 


Can. 


The  spinal  cord  develops  in  an  essentially  uniform  manner  except  at  its  caudal 
extremity,    the   development    of    which    is  Part.  G.    B.       D.H. 

arrested  before  differentiation  sets  in. 
The  undifferentiated  extremity  becomes 
the  filum  terminale  of  the  adult.  Typi- 
cally, the  three  primary  layers,  ectoglia, 
cinerea,  and  ependyma,  are  early  differ- 
entiated. The  ectoglia  increases  in  thick- 
ness and  receives  many  nerve-fibers, 
which  run  for  the  most  part  lengthwise, 
and  is  thus  transformed  into  the  white 
matter  of  the  adult  cord.  The  cinerea 
becomes  the  gray  matter  and  gradually 
assumes  the  characteristic  adult  outline  in 
cross-sections  (dorsal  and  ventral  horns). 
The  ependymal  layer  remains  thin,  its 
cells  contributing  to  the  formation  of  the 
neuroglia  frame-work.  During  the  growth 
of  the  cord,  the  ventral  zones  enlarge 
rapidly  and  each  projects  downward, 
leaving  a  notch  between  them  (compare 
Fig.  213,  Sp.c).  The  notch  is  closed  on 
its  dorsal  side  by  the  thin  floor-plate  by  which  the  two  ventral  zones  are  con- 
nected across.  The  ventral  expansions  increase  and  the  notch  is  thus  trans- 


Fiss. 


V.H. 


FIG.  40. — TRANSVERSE  SECTION  OF  SPINAL  CORD  OF  A 
HUMAN  EMBRYO  OF  32  MM. 

B,  Burdach's  column.  Can,  Central  canal.  D.H, 
Dorsal  horn.  Fiss,  Ventral  fissure.  G,  Goll's 
column.  Part,  Dorsal  partition.  R.d,  Dorsal 
nerve  root.  V.H,  Ventral  horn. 


76  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

formed  into  the  ventral  fissure,  figure  40,  Fiss.,  filled  with  mesenchyma  or,  in  the 
adult,  by  vascularized  connective  tissue.  Further,  the  inner  surfaces  of  the  dorsal 
zone  meet  and  unite,  thus  obliterating  the  dorsal  portion  of  the  original  central 
canal,  and  forming  out  of  the  fused  ependymal  layers  the  permanent  posterior 
partition,  figure  40,  Part.  The  central  canal  of  the  adult,  figure  40,  Can,  corresponds, 
therefore,  to  the  ventral  part  only  of  the  original  canal. 

Plakodes. 

Plakodes  are  small  circumscribed  thickenings  of  the  ectoderm,  which  contribute 
to  the  development  of  the  olfactory,  visual,  and  auditory  organs.  There  are  accord- 
ingly three  pairs  of  them.  They  resemble  one  another  very  closely  in  original  ap- 
pearance and  their  early  history.  A  plakode,  figure  41,  Plk,  is  a  rounded  area  of 

-. 

EC.  Plk.  Seg. 


Mes.  Ent.  Endo.  Ao. 


FIG.  41. — CHICK  EMBRYO  OF  THIRTEEN  SEGMENTS.     SAGITTAL  SERIES  1452,  SECTION  52. 

Ao,  Aorta.     EC,  Ectoderm.     Endo,  Endothelium.     Ent,  Entoderm.     Mes,  Mesoderm.     Plk,  Auditory  plakode. 

Seg,  Mesodermic  somite.     X  100  diams. 

syncytial  ectoderm  of  small  dimensions,  several  times  as  thick  as  the  surround- 
ing ectoderm,  with  which  it  merges  by  a  rapid  transition  in  the  diameter  of  the 
layer.  Soon  after  its  appearance  it  becomes  invaginated.  By  preserving  this  con- 
dition the  olfactory  plakode  forms  the  olfactory  pit.  In  other  cases  the  invagina- 
tion  deepens,  then  closes  over,  and  the  vesicle  thus  formed  separates  from  the  over- 
lying ectoderm.  The  vesicle  formed  by  the  visual  plakode  becomes  the  lens  of  the 
eye.  The  vesicle  formed  by  the  auditory  plakode  becomes  the  otocyst.  The  ulti- 
mate development  of  these  organs  is  considered  in  the  three  sections  next  following. 

The  Nasal  Pits  and  Olfactory  Nerves. 

The  olfactory  plakodes  arise  as  a  symmetrical  pair  of  thickenings  underneath 
the  fore-brain.  They  soon  become  invaginated,  forming  two  shallow  depressions  just 
in  front"  of  the  mouth,  as  is  well  shown  in  figure  165.  As  development  progresses, 
the  depressions  deepen  and  remain  lined  throughout  by  the  thickened  ectoderm 
(Fig.  194,  No).  The  orifices  may  be  temporarily  closed  by  the  coalescence  of  the 
epithelium.  The  olfactory  pits  acquire  a  secondary  opening  into  the  oral  cavity 
(Fig.  219).  By  the  expansion  of  this  cavity,  the  nasal  chamber  proper  of  the  adult 


THE  EYE.  77 

is  produced.  The  form  becomes  complicated  by  the  development  of  the  turbinal 
folds  (Fig.  212)  and  later  by  the  addition  of  outgrowths  from  each  nasal  pit  to 
form  the  accessory  sinuses  of  the  adult  nasal  cavity.  The  tissue  between  the  two 
nasal  pits  forms  the  septum  (Figs.  220  and  212,  Sept}.  The  epithelium  of  each 
pit  sends  a  special  invagination  into  the  nasal  septum  to  make  the  gland-like  struc- 
ture known  as  Jakobson's  organ  (Fig.  219,  Jk.o).  It  will  be  seen  from  the  above 
that  the  extent  of  the  nasal  epithelium  becomes  very  great,  but  only  a  small  area 
is  concerned  in  the  formation  of  the  olfactory  organ  proper.  This  area  underlies 
the  olfactory  bulb  of  the  fore-brain,  and  is  termed  the  olfactory  epithelium.  The 
cells  of  this  become  elongated  and  acquire  boundaries  from  one  another  so  as  to 
make  a  cylinder  epithelium.  Some  of  the  cells  become  the^o-called  olfactory  cells, 
which  are  more  or  less  isolated  and  separated  from  one  another  by  the  intervening 
supporting  cells.  The  olfactory  cells  develop  on  their  free  ends  a  few  small  pro- 
jecting hairs,  and  from  their  basal  ends  a  single  thread-like  prolongation,  which 
becomes  a  fiber  of  the  olfactory  nerve.  This  fiber  penetrates  the  olfactory  bulb, 
and  there  has  its  terminal  arborization,  which  enters  into  special  relations  with  the 
mitral  cells  of  the  bulb.  All  the  fibers  of  the  olfactory  nerve  arise  in  this  way; 
hence  the  nerve — so  far  as  its  development  is  concerned — is  unique  in  vertebrates. 
It  differs  permanently  from  all  other  nerves  in  that  its  fibers  never  acquire  any 
medullary  sheaths,  because  no  cells  migrate  from  the  medullary  wall  or  brain  into 
this  nerve  as  they  do  into  all  others. 

Later,  the  complete  separation  of  the  nasal  and  oral  cavities  is  accomplished 
by  the  development  of  the  palate  shelves  which  grow  out  from  the  walls  of  the  oral 
cavity  until  they  meet  in  the  median  line  and  unite  with  the  lower  edge  of  the 

nasal  septum,  as  described  and  illustrated  in  Chapter  VI. 

• 

The  Eye. 

The  eye  has  a  complicated  history.  The  optic  nerve  and  retina  arise  from  the 
medullary  tube  (brain).  The  lens  arises  from  .the  visual  plakode.  The  remaining 
structures  of  the  eye  are  of  mesodermal  origin. 

Optic  Vesicles.—  The  optic  vesicles  arise  very  early  from  the  extreme  cephalic  end 
of  the  medullary  tube,  as  two  lateral  outgrowths  (Fig.  131),  each  of  which  soon  ap- 
pears quite  as  large  as  the  central  portion  of  the  medullary  tube  which  produces 
it  (see  Fig.  133).  The  central  portion  of  the  tube,  however,  grows  much  more 
rapidly  than  the  optic  vesicles  in  order  to  form  the  brain.  The  distal  portion  of 
the  optic  vesicle  expands,  and  we  thus  get  the  condition  indicated  in  figure  154,  Op. 
The  vessel  may  now  be  said  to  be  stalked.  The  stalk  is  the  anlage  of  the  optic 
nerve.  The  larger  distal  portion  of  the  optic  vesicle  gives  rise  to  the  retina.  The 
optic  vesicle  comes  in  contact  with  the  ectoderm,  and  over  the  area  of  contact  the 
visual  plakode  is  differentiated.  In  the  next  stage  the  differentiation  between  the 
eyeball  and  optic  nerve  is  clearly  seen  (Fig.  154).  This  is  accomplished  by  modifi- 
cations in  the  distal  portion  of  the  optic  vesicle.  Its  outer  wall  becomes  invagi- 


78  THE  EARI.Y  DEVELOPMENT  OF  MAMMALS. 

nated,  producing  a  cup,  the  wall  of  which  is  necessarily  double.  The  layer  next  the 
cavity  of  the  cup  increases  in  thickness  and  becomes  the  retina  proper.  The  other 
layer  remains  thin  and  becomes  charged  with  pigment,  and  is  transformed  into  the 
so-called  pigment  layer  of  the  retina.  The  opening  of  the  cup  is  filled  with  the 
lens  (compare  also  Fig.  17).  In  accordance  with  the  fact  that  the  optic  nerve 
and  retina  are  derived  from  the  wall  of  the  medullary  tuber  we  find  that  their  dif- 
ferentiation is  essentially  similar  to  that  of  the  central  nervous  system.  They 
develop  neuroglia  and  nerve-cells  with  neuraxons.  It  is  only  by  keeping  in  mind 
these  facts  that  the  histogenesis  of  the  adult  structure  of  the  retina  can  be 
understood. 

Lens. — The  visual  fllakode,  which,  it  will  be  remembered,  is  at  first  in  contact 
with  the  wall  of  the  optic  vesicle,  is  early  invaginated  (Fig.  153,  L).  The  invagi- 
nation  closes  over,  making  the  lentic  vesicle;  this  rapidly  separates  from  the  over- 
lying ectoderm,  which  ultimately  becomes  the  corneal  epithelium.  The  wall  of  the 
vesicle  which  is  nearest  the  retina  and  farthest  from  the  epidermis  rapidly  thickens 
and  forms  the  main  substance  of  the  lens,  at  the  same  time  obliterating  the  cavity 
of  the  vesicle. 

Mesoderm. — The  mesoderm  produces  the  choroid  and  sclerotic  coats  of  the 
adult  eyeball,  the  connective  tissue  of  the  iris  and  of  the  cornea,  the  lining  epi- 
thelium of  the  anterior  chamber  of  the  eye,  and  the  muscles  which  move  the 
eye-ball. 

For  further  details  as  to  the  history  of  the  eye  see  page  331. 

The  Otocyst. 

The  otocyst  arises  by  the  invagination  of  the  auditory  plakode  (Fig.  41,  Plk) 
to  form  the  auditory  pit  "(Fig.  152,  Of),  which  soon  closes,  forming  an  epithelial 
vesicle  which  quickly  loses  its  connection  with  the  overlying  ectoderm.  The  vesicle 
or  otocyst  is  the  anlage  of  the  membranous  labyrinth  of  the  ear.  It  lies  close  to 
the  wall  of  the  myelencephalon  (Fig.  191,  Of)  between  the  ninth  nerve  and  the 
ganglion  complex  of  the  seventh-eighth  nerves.  It  becomes  pear-shaped,  the  nar- 
row end  pointing  dorsally.  It  soon  develops  a  special  prolongation  (Fig.  42, 
D.endo)  which  extends  dorsad  near  the  brain-wall  and  is  known  as  the  ductus 
endolymphaticus.  The  ductus  persists  throughout  life. 

The  ventral  end  (Cock)  of  the  otocyst  begins  to  elongate  very  early,  and  is 
converted  gradually  into  a  very  long  spiral  epithelial  tube,  the  scala  media  cochlea 
of  the  adult.  The  'dorsal  summit,  S.C,  of  the  otocyst  is  transformed  into  the 
semicircular  canals.  The  middle  part  of  the  original  vesicle  also  undergoes  remark- 
able changes  of  form  to  produce  the  utriculus,  which  opens  into  the  anterior  and 
external  semicircular  canals;  the  canalis  reuniens,  which  leads  to  the  cochlea;  and 
the  saccidus,  a  blind  pouch  on  the  anterior  side  of  the  canalis  reuniens. 

The  entire  epithelial  labyrinth  becomes  surrounded  by  a  loose  mesenchyma, 
which  again  is  surrounded  by  denser  tissue  which  forms  the  cartilaginous  periotic 


THE  EARLY  HISTORY  QF  THE  MESODERM. 


79 


capsule.     The  capsule  enters  into  the  formation  of  the  cranium,  and  when  it  ossifies 
forms  also  the  so-called  bony  labyrinth. 

The  auditory  ganglion  fuses  very  early  with  the  cephalad  wall  of  the  otocyst, 
so  that  in  sections  no  boundary  between  the  ganglion  and  the  epithelium  can  be 
distinguished.  The  upper  part  develops  into  the  vestibular,  the  lower  into  the  coch- 
lear  ganglion. 


R.VIII.  D.endo. 


S.C. 


R.VI1. 


Cock. 


N.VII. 


FIG   42. — PIG  EMBRYO  OF  12  MM.     SERIES  5.     RECONSTRUCTION  IN  WAX  OF  THE  OTOCYST,  WITH  PART  OF  THE 

BRAIN-WALL  AND  THE  SEVENTH  AND  EIGHTH  NERVES.     BY  G.  C.  COE  and  W.  W.  BEHLOW.    VIEW  FROM 

THE  CAUDAD  SIDE. 
Br,     Brain- wall.     Cock,    Cochlear  anlage.      D.endo,    Ductus    endolymphaticus.     N.VII,    Seventh    nerve.     R. 

VII,  Root  of  seventh  nerve.     R.  VIII,  Root  of  eighth  nerve.     S.C,  Region  from  which  the  semicircular 

canals  are  formed.     X  100  diams. 


The  Early  History  of  the  Mesoderm. 

Concerning  the  precise  origin  and  early  development  of  the  mesoderm  authori- 
ties are  by  no  means  agreed,  and  in  the  interpretations  offered  there  has  been  more 
of  hypothesis  than  of  observation.  The  most  accurate  observations  have  so  far 
been  made  on  the  elasmobranchs,  lizards,  and  chick.  In  these  -forms  the  entoderm 
(or  segmenting  yolk)  in  the  neighborhood  of  the  primitive  streak  produces  cells 
which  take  their  place  so  as  to  form  a  layer  next  to  the  entoderm.  This  layer 
gradually  becomes  more  and  more  distinct  until  it  can  be  definitely  recognized  as 
a  separate  layer,  the  mesoderm.  It  is  probable  that  a  similar  process  goes  on  in 
amphibia  and  in  mammals,  so  that  it  is  safe  to  say  that  the  mesoderm  probably 
arises  by  this  process,  which  we  calh  delamination,  in  all  vertebrates.  In  its  first 


80 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


stage  the  mesoderm  has  no  distinct  boundary  against  the  underlying  entoderm.  It 
is  thickest  in  the  neighborhood  of  the  primitive  streak  and  thins  out  from  that  in 
all  directions.  It  very  early  comprises  two  easily  recognizable  classes  of  cells.  One 
of  these  forms  a  more  or  less  distinct  layer  next  to  the  yolk,  and  so  distributes 
itself  as  to  form  a  network  of  cavities  of  which  these  cells  become  the  boundaries, 
thus  developing  the  first  blood-vessels.  The  cells  which  form  them  constitute  the 
angioblast.  A  portion  of  the  angioblastic  cells  comes  to  lie  in  the  cavities  of  these 
primitive  blood-vessels  and  is  transformed  into  the  first  red  blood-corpuscles  of  the 
embryo.  The  second  class  of  cells  constitutes  the  mesoderm  proper^  and  forms  a 
more  continuous  sheet  of  undifferentiated,  somewhat  closely  compacted  cells,  ex- 
tending out  from  the  primitive  streak  and  lying  between  the  angioblast  and  the 
ectoderm. 

The  Expansion    of  the  Mesoderm. — After  the-  mesoderm  is  once  formed  as  a  dis- 
tinct layer,  it  seems  to  have  no  longer  any  connection  with  the  entoderm  or  ecto- 


A 


FIG.  43. — THREE  DIAGRAMS  OF  EMBRYONIC  AREAS  OF  CHICKS  TO  SHOW  THE  GROWTH  OF  THE  MESODERM. 

The  mesoderm  is  indicated  by  vertical  shading,  the  area  opaca  by  horizontal  shading.     A.o,  Area  opaca.     A.p, 

Area  pellucida.     mes,  Mesoderm.     pr,  Primitive  streak. — (After  Duval.) 


derm,  except  in  the  axial  line.  Its  further  expansion  is  due  to  the  proliferation  of 
its  own  cells.  During  this  early  expansion  the  mesoderm  assumes  in  all  amniota  a 
definite  and  characteristic  series  of  outlines.  It  is  at  first  pear-shaped  (Fig.  43,  A), 
the  anterior  end  being  pointed.  It  extends  a  short  distance  only  in  front  of  the 
primitive  streak  and  is  widest  a  little  distance  behind  the  area  pellucida  (Ap].  (For 
a  description  of  the  area  pellucida  see  Chapter  V.)  The  condition  in  the  chick  at 
about  the  twentieth  hour  of  incubation  is  indicated  by  figure  43,  B,  drawn  on  the 
same  scale  as  A,  and  at  the  close  of  the  first  day  by  figure  43,  C.  In  the  last  stage 
figured  it  will  be  noticed  that  the  mesoderm  is  expanding  unequally  in  front,  hav- 
ing sent  out  two  lateral  wings  which  leave  a  median  space  between  them  without  meso- 
derm. These  wings  continue  their  growth,  and  finally  meet  in  front,  so  that  in  the 
anterior  part  of  the  area  pellucida  there  is  a  small  tract  without  any  mesoderm, 
although  it  is  completely  enclosed  by  mesoderm.  This  tract  is  the  pro-amnion.  The 


THE  EARLY  HISTORY  OF  THE  MESODERM.  81 

actual  expanding  edge  of  the  mesoderm  is  quite  irregular.  The  regularity  shown  in 
figure  43  is  entirely  diagrammatic. 

The  extent  of  the  growth  of  the  mesoderm  over  the  extra-embryonic  region  of 
the  mammalian  blastodermic  vesicle  is  very  variable.  Usually  it  extends  completely 
around  the  vesicle,  but  in  some  cases,  as  in  the  rabbit,  only  part  way  (compare 
page  52). 

The  Origin  of  the  Ccelom. — The  next  step  in  the  differentiation  of  the  middle 
germ-layer  is  the  appearance  of  two  slit-like  cavities  in  it,  one  on  each  side. 
These  cavities  do  not  extend  across  the  median  line,  for  when  they  appear  there  is 
no  mesoderm  in  the  median  line  of  the  embryo.  The  ccelom  is  the  anlage  of  the 
body-cavity,  and  in  part  persists  in  the  adult  as  the  pericardial,  pleural,  and  ab- 
dominal cavities.  Certain  parts  of  its  walls  share  in  the  production  of  muscles 
and  of  the  excretory  organs.  The  complete  history  of  the  ccelom  is  very  complex. 
As  the  coelomatic  cavities  appear,  the  cells  bounding  them  take  on  a  distinctly 
epithelial  character.  This  limiting  layer  is  termed  the  mesothelium. 

The  earliest  phases  in  the  development  of  the  ccelom  have  been  exactly  fol- 
lowed only  in  a  very  few  instances.  In  these  it  has  been  found  that  numerous 
fissures  appear  in  the  mesoderm  and  unite  themselves  so  as  to  form  a  network  of 
channels  which  grow,  and  produce  by  their  fusion  the  ccelom.  The  fusion  occurs 
so  that  two  cavities  are  developed,  one  on  either  side,  and  parallel  with  the  axis  of 
the  embryo.  As  the  head  of  the  embryo  grows  the  two  cavities  grow  into  its 
cervical  end,  following  the  penetration  of  the  mesoderm,  and  unite  so  as  to  form 
below  the  developing  pharynx  a  single  median  cavity,  the  anlage  of  the  future  peri- 
cardial cavity.  In  the  Sauropsida  and  in  many  mammals  the  pericardial  ccelom 
merges  into  two  large  expansions  of  the  body-cavity  which  lie  just  alongside  of 
the  head  of  the  embryo  and  are  known  as  the  amnio-cardiac  vesicles  (Fig.  131, 
A.c.v).  (Compare  also  the  account  of  the  splanchnocele,  page  87.) 

There  are  very  great  variations  in  the  development  of  the  ccelom  in  mammals. 
In  some  cases  the  coelom  grows  so  as  to  appear  at  an  early  stage  in  the  body  of 
the  embryo  (Fig.  37).  In  other  cases  it  is  developed  in  the  entire  extra-embryonic 
region  of  the  blastodermic  vesicle  before  it  is  developed  in  the  embryo  proper. 
This  condition  has  been  observed  in  primates,  including  man.  It  results  in  the 
formation  of  a  layer  of  mesoderm  surrounding  the  yolk-sac,  and  another  layer 
underlying  the  extra-embryonic  ectoderm,  with  a  wide  ccelomate  space  between  the 
two  mesodermic  layers.  This  space  we  call  the  extra-embryonic  ccelom.  These 
relations  are  illustrated  in  figure  45. 

As  soon  as  the  ccelom  has  appeared  the  mesoderm  is. divided  into  two  layers,  an 
outer  and  an  inner.  The  outer  layer  is  in  close  contact  with  the  ectoderm.  It  is 
called  the  somatic  mesoderm.  The  inner  layer  is  in  close  contact  with  the  entoderm; 
it  includes  the  entire  angioblast,  there .  being  in  early  stages  no  blood-vessels  or 
blood  in  the  somatic  mesoderm.  The  inner  layer  is  called  the  splanchnic  mesoderm. 


82 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


/Ent 


EC 


Somatopleure  and  Splanchnopleure. 

The  somatic  mesoderm,  together  with  the  overlying  ectoderm,  constitutes  the 
somatopleure  or  primitive  body-wall.  The  splanchnic  mesoderm,  together  with  the 
underlying  entoderm,  constitutes  the  Splanchnopleure.  The  somatopleure  and  Splanch- 
nopleure are,  to  a  large  degree,  the  elementary  anatomical  parts  out  of  which  the 
adult  structure  is  produced.  Although  they  each  comprise  cells  belonging  to  two 
germ-layers,  they  nevertheless  develop  each  almost  as  a  unit,  the  cells  of  the  two 
germ-layers  entering  into  intimate  co-operation  with  one  another  in  the  differen- 
tiation of  organs.  In  both  somatopleure  and 
Splanchnopleure  it  is  convenient  to  distinguish  two 
main  regions;  namely,  the  embryonic,  which  enters 
into  the  constitution  of  the  embryo  proper,  and 
the  extra-embryonic,  which  enters  into  the  forma- 
tion of  the  so-called  appendages  of  the  embryo, 
that  is  to  say,  of  parts  which  exist  during  em- 
bryonic life,  but  are  lost  at  the  time  of  birth, 
and  take  no  share  in  the  permanent  body. 

In  the  primitive  type  of  vertebrate  develop- 
ment there  are  no  embryonic  appendages.  This 
condition  is  illustrated  by  figure  44,  which  is  a 

transverse  section  of  a  young  stage  of  an  axolotl. 
FIG.    44. — TRANSVERSE    SECTION    OF    AN  * 

EARLY  STAGE  OF  AN  AXOLOTL.  This    may    be    readily    compared    with    a    blasto- 

Ec,  Ectoderm,     mes,    Mesoderm.     Md,    dermic    vesicle    of   a  mammal,   if  we   imagine  the 

Medullary    groove.     Ch,    Notochord.     mags     of          }k     Qr     entoderm     reduced     to    a    single 
Ent,     Entoderm.      Yk,     Yolk.      Ach, 
•  Archenteron  or  primitive  entodermal      laJer   of  cells-      We    can   then  easily   distinguish   the 

cavity.— (After  Bellond.)  ectoderm   and    the   underlying  somatic  mesoderm, 

which     together    completely    enclose    the    section. 

The  splanchnic   mesoderm   lies   close   against   the   yolk  and    is    separated    from    the 
somatic  by  the  intervening  coelom. 

The  general  homologies  of  this  primitive  type  of  vertebrate  embryos  with  the 
type  which  we  find  in  the  amniota  may  be  readily  grasped  by  the  aid  of  the  ac- 
companying diagrams  (Fig.  45),  which  are  based  somewhat  on  the  processes  as 
actually  found  in  the  chick.  The  embryonic  structures  properly  so  called  are  dis- 
tinguished by  shading.  The  yolk-sac  is  large  and  more  or  less  a  separate  structure 
from  the  embryo.  It  is  surrounded  by  a  layer  of  mesoderm  represented  by  a  dotted 
line.  In  the  direction  of  the  embryo  the  mesoderm  has  continued  to  form  part  of 
the  wall  of  the  intestinal  canal,  In;  hence  we  may  say  that  the  Splanchnopleure 
forms  the  wall  of  the  primitive  intestinal  canal  and  of  the  yolk-sac.  The  yolk-sac 
represents  a  lower  portion  of  the  Splanchnopleure.  It  can  be  readily  seen  that  we 
may  compare  it  with  the  condition  noted  in  the  newt,  and  have  to  deal  funda- 
mentally with  a  question  of  relative  proportions.  The  somatopleure,  Som,  enters  into 
the  formation  of  the  embryo  itself,  but  it  also  extends  beyond.  Its  disposition  be- 


SOMATOPLEURE  AND  SPLANCHNOPLEURE. 


83 


comes  complicated  in  the  amniota  by  the  formation  of  the  amnion  itself.  We  shall 
consider  here  only  what  is  looked  upon  as  the  primitive  method  of  the  production 
of  the  amnion,  and  note  only  that  the  exact  steps  of  the  process  are  considerably 
modified  in  many  mammals,  in  connection  with  the  early  modifications  which  the 
ovum  undergoes  in  order  to  secure  its  attachment  to  the  walls  of  the  uterus  (see 
the  section  on  the  trophoderm).  The  somatopleure  forms  two  folds,  one  on  each  side 
of  the  embryo.  These  folds  arch  up  over  the  back  of  the  embryo.  The  inner  leaf 
or  part  of  each  fold  is  the  anlage  of  the  amnion,  Am.  It  consists  of  a  layer  of  ecto- 
derm next  to  the  embryo,  and. a  layer  of  mesoderm,  represented  by  the  dotted  line, 


Cho 


Cho. 


FIG.  45. — GENERALIZED  DIAGRAM  OF  AN  AMNIOTE  VERTEBRATE  EMBRYO. 

The  first  figure  shows  the,  condition  before,  the  second  after,  the  separation  of  the  amnion  from  the  chorion.  Am, 
Amnion.  Cho,  Chorion.  Cae,  Ccelom.  In,  Intestinal  canal.  Som,  Somatopleure.  Spl,  Splanchnopleure. 
Yolk,  Yolk-sac. 

turned  away  from  the  embryo.  The  remaining  portion  of  the  extra-embryonic  so- 
matopleure, Cho,  extends  around  both  the  amnion  and  yolk-sac,  forming  a  mem- 
brane called  the  chorion,  which  likewise  consists,  of  course,  of  ectoderm,  which, 
however,  faces  away  from  the  embryo,  and  of  mesoderm  (dotted  line),  which  is 
turned  toward  the  embryo.  As  regards  the  embryo,  therefore,  the  position  of  the 
two  germ-layers  in  the  amnion  is  reversed  in  the  chorion.  The  two  folds  continue 
to  grow  until  they  meet  above  the  back  of  the  embryo  and  unite.  The  amnion 
(Fig.  45,  Am)  has  thus  become  a  closed  membrane  surrounding  the  embryo,  and 
the  chorion,  Cho,  has  become  a  closed  membrane  surrounding  the  amnion,  the 
embryo,  and  the  yolk-sac. 

By  the  processes  indicated  we  have  produced  an  embryo  with  its  three  primary 
appendages — the  chorion,  amnion,  and  yolk-sac.  To  these  there  is  to  be  added  a 
fourth  appendage,  the  allantois,  which  also  begins  its  development  very  early,  and 
arises  as  a  hollow  outgrowth  from  the  under  side  of  the  caudal  end  of  the  embryo 
and  expands  into  the  extra-embryonic  ccelom  or  space  between  the  yolk-sac  and 
the  chorion. 


84  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

The  Embryonic  Ccelom. 

In  the  body  of  the  embryo  proper  the  ccelom  acquires  a  very  complicated  dis- 
position. It  forms,  first,  a  series  of  small  cavities  alongside  of  the  medullary  tube. 
The  walls  of  these  cavities  are  termed  the  somites.  A  pair  of  somites  mark  out  a 
primitive  segment.  It  forms,  secondly,  two  large  main  cavities,  which  partially  unite 
in  later  stages  on  the  ventral  side  of  the  embryo,  the  primitive  segments  lying  more 
on  the  dorsal  side.  These  two  large  ccelom  spaces  constitute  the  splanchnocele,  a 
term  which  has  reference  to  the  fact  that  this  space  surrounds  the  splanchnic  vis- 
cera. Finally,  it  forms  a  series  of  so-called  head-cavities,  of  which  there  are  proba- 
bly always  three  on  each  side  of  the  head.  The  walls  of  these  head-cavities  in 
part  produce  the  muscles  of  the  eye.  We  must  now  consider  the  development  of 
these  divisions  of  the  ccelom  in  the  order  indicated. 

The  Primitive  Segments. — A  segment  consists  of  a  pair  of  cavities  symmetrically 
placed  and  bounded  by  mesothelium.  The  cavities  are  portions  of  the  embryonic 
coelom.  For  convenience  of  description  the  term  somite  is  applied  to  one  of  the 
pair  of  structures  which  constitute  a  whole  segment.  The  somites  appear  very  early; 
the  first  pair  can  be  recognized  in  the  chick  after  twenty  to  twenty-two  hours' 
incubation;  in  the  rabbit,  at  the  beginning  of  the  eighth  day.  In  both  cases  the 
medullary  groove  is  still  nowhere  closed.  In  amniote  embryos,  just  before  the  first 
segment  appears,  the  mesoderm  on  either  side  of  the  axial  line  is  considerably 
thicker  than  farther  away  from  it.  We  can,  therefore,  distinguish  two  zones,  namely, 
the  thicker  segmental  zone  near  the  axis,  and  the  thinner,  but  much  wider  lateral 
or  parietal  zone  (Figs.  129,  131).  The  first  step  in  the  formation  of  the  first  seg- 
ment is  a  loosening  of  the  cells  in  the  segmental  zone,  along  a  narrow  transverse 
line.  In  the  chick  this  occurs  about  0.14  mm.  in  front  of  the  primitive  streak,  at 
a  time  when  only  a  portion  of  the  medullary  groove  is  formed.  Very  soon  there 
appears,  close  by,  a  second  similar  transverse  loosening  of  the  cells.  The  mesoderm 
of  the  segmental  zone  is  thus  cleft  twice,  the  mesodermic  cells  between  the  two 
clefts  constituting  the  first  somite,  which  is  somewhat  cuboidal  in  form.  The  first 
segment  appears  in  what  later  becomes  the  occipital  region.  All  further  segments 
are  formed  successively  in  a  similar  manner  behind  the  first,  the  series  growing  by 
additions  caudad.  The  segments  differ  somewhat  one  from  another  in  the  details 
of  their  development.  The  primitive  somites,  owing  to  their  form  and  their  prox- 
imity to  the  anlage  of  the  central  nervous  system,  were  taken  by  early  embryologists 
to  be  the  beginnings  of  the  vertebrae,  and  were,  therefore,  called  the  proto-vertebrcE. 
This  name  is  still  used,  although  the  idea  upon  which  it  was  based  is  known  to 
be  erroneous,  because  the  primitive  segments  form  much  more  than  the  vertebrae. 

The  association  in  time  of  the  development  of  the  medullary  groove  and  primi- 
tive segments  is  important.  By  the  formation  of  the  groove  the  space  between 
the  ectoderm  and  entoderm  alongside  the  groove  is  increased,  and  it  is  this  space 
which  gives  the  mesoderm  the  opportunity  to  grow  in  thickness  so  as  to  form  the 
segmental  zone  next  to  the  medullary  groove. 


THE  EMBRYONIC  CCELOM.  85 

In  the  amniota  when  the  somites  are  first  formed  they  display  usually  no  actual 
cavity,  but  we  must  consider  that  one  is  morphologically  present,  since  we  can 
easily  observe  the  line  of  contact  between  the  opposite  walls  of  the  segments.  As 
observed  in  transverse  sections,  the  somites  are  seen  to  become  triangular  in  outline. 
The  base  of  the  triangle  extends  along  the  side  of  the  medullary  canal;  the  apex 
of  the  triangle  lies  next  to  the  splanchnocele,  and  at  the  point  of  the  triangle  the 
somatic  and  splanchnic  mesoderm  separate  widely  from  one  another.  Very  soon 
the  apex  of  the  triangle  forms  a  narrow  piece  (Fig.  46,  N),  which  is  known  com- 
monly as  the  nephrotome  or  intermediate  cell-mass.  While  the  nephrotome  is  being 
marked  off  the  proximal  portion  of  the  segment  enlarges,  the  cells  assume  a  more 
distinctly  epithelial  character  (Fig.  46,  My),  enclosing  a  considerable  space,  which, 
however,  is  completely  filled  by  a  mass  of  cells,  C,  which  arise  by  a  proliferation 

•«/  «JS«>*S'_<•il3t£lt**S,,•^iQJ,  _       SOITl 


Spl 


FIG.  46. — TRANSVERSE  SECTION  OF  THE  MESODERM  OF  A  CHICK  EMBRYO  WITH  ABOUT  EIGHTEEN  SEGMENTS. 
Only  the  mesoderm  of  one  side  has  been  drawn.  The  section  passes  through  a  recently  formed  segment.  My, 

Secondary  segment.     C,  Core  of  the  segment.     W.d,  Wolffian  duct.     N,  Nephrotome.     Cos,  Ccelom.     Som, 

Somatic  mesoderm.     Spl,  Splanchnic  mesoderm.      X  227  diams. 

of  the  cells  from  the  lower  side  of  the  segment.  The  line  around  this  mass  of 
cells  marking  it  off  from  the  other  wall  of  the  segment  indicates  the  morphological 
cavity.  In  the  sheep  and  the  chick  it  has  been  observed  that  the  cavities  of  the 
first  four  segments  can  be  traced  through  the  nephrotome  to  the  splanchnocele. 
This  represents  a  primitive  condition,  one  which  we  find  in  all  the  segments  of 
some  of  the  lower  vertebrates.  Did  we  know  the  development  of  the  amniota 
only,  we  should  not  have  been  able  to  identify  the  cavity  of  the  somite  as  mor- 
phologically a  portion  of  the  crelom.  The  development  in  fishes  shows  conclusively 
that  it  must  be  so  regarded. 

The  Separation  of  the  Nephrotome. — The  nephrotome  early  loses  its  connection 
on  the  one  side  with  the  enlarged  central  portion  of  the  somite,  and  on  the  other 
with  the  mesodermic  walls  of  the  splanchnocele,  so  that  each  nephrotome  forms 
a  little  mass  of  cells  isolated  from,  but  in  definite  topographical  relation  to,  the 
other  parts  of  the  mesoderm.  It  may  be  noted  that  during  these  early  stages  one 
can  always  find  the  anlage  of  the  Wolffian  duct  on  the  ectodermal  side,  and  on  the 
entodermal  side  the  anlage  of  a  blood-vessel.  Very  soon  the  nephrotome  assumes 


86 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


a  rounded  form,  and  a  cavity  appears  in  its  interior;  it  is  then  often  called  a 
segmental  vesicle  (Fig.  47,  Nephr}.  The  exact  details  of  the  process  by  which  the 
nephrotome  is  separated  from  the  other  parts  of  the  middle  germ-layer  have  not 
yet  been  carefully  studied.  Each  nephrotome  is  the  anlage  of  one  of  the  excretory 
tubules  of  the  Wolffian  body.  It  elongates  into  a  tubule,  which  takes  an  S-shape, 
and  extends  in  the  transverse  plane  of  the  body.  The  lateral  end  of  the  tubule 
unites  with,  and  acquires  an  opening  into,  the  Wolffian  duct.  The  median  end 
expands  and  produces  a  nephric  corpuscle,  with  the  characteristic  glomerulus  and 
capsule. 


Am. 


Som. 


Ao. 


Nch. 


FIG.  47. — SECTION  OF  A  VERY  YOUNG  CAT  EMBRYO.    TRANSVERSE  SERIES  413,  SECTION  181. 
Am,  Amnion.     A o,  Aorta.     Md,  Medullary  tube  (spinal  cord).     My,  Outer,  My',  inner  wall  of  primitive  segment 
Nch,  Notochord.     Nephr,  Nephrotome   (segmental  vesicle).    Som,  Somatopleure.    Spl,   Splanchnopleure. 
Ve,  Blood-vessel.     W.D,  Wolffian  duct.     X  50  diams. 

The  portion  of  the  somite  which  is  isolated  by  the  formation  of  the  nephro- 
tome lies,  of  course,  next  to  the  medullary  canal.  The  term  primitive  segment  (as 
also  proto-vertebra)  is  often  applied  to  this  structure  as  well  as  to  the  original 
somite  before  the  separation  of  the  nephrotome,  but  it  would  be  better  to  refer  to 
it  as  the  secondary  somite*  The  secondary  somite,  when  first  formed,  appears  more 
or  less  nearly  square  in  surface  views,  and  triangular  in  cross-sections.  As  the 
medullary  canal  grows,  so  does  the  secondary  somite,  and  it  becomes,  therefore, 
somewhat  elongated  in  its  dorso-ventral  diameter.  After  this  change  in  its  shape 
we  can  distinguish  in  transverse  sections  of  an  embryo  (Fig.  38)  the  outer  wall, 
which  lies  under  the  ectoderm,  and  an  inner  wall,  which  lies  toward  the  medullary 
canal  and  notochord.  In  the  further  history  of  the  somite  we  can  distinguish  the 
following  steps:  first,  the  production  of  the  dermatome  (cutis  plate)  with  the  ac- 
companying transformation  of  a  portion  of  the  cells  of  the  inner  wall  of  the -seg- 
ment into  the  mesenchyma;  next,  the  production  of  the  true  muscle-plate;;  thirdly, 

*  This  is  a  new  term,  here  proposed  for  the  first  time. 


THE  EMBRYONIC  CCELOM.  87 

the  breaking-up  of  the  outer  wall  of  the  myotome.  These  portions  are  sufficiently 
described  in  the  practical  part,  Chapter  V. 

The  Splanchnocele. — The  splanchnocele  makes  its  first  appearance  in  the  parietal 
zone  of  the  mesoderm  in  the  manner  above  described  (Figs.  45  A  and  B,  Coe). 
It  rapidly  increases  in  size,  so  that  a  considerable  space  separates  the  somatic  from 
the  splanchnic  mesoderm,  as  shown  in  figures  160  and  163.  When  it  first  ap- 
pears, it  is  a  narrow  fissure.  It  rapidly  widens,  extends  toward  the  axis  until  it 
almost  reaches  the  primitive  segments,  and  also  spreads  out  laterally  into  the  so- 
called  extra-embryonic  region.  As  above  stated,  the  rate  and  extent  of  its  extra- 
embryonic  development  vary  greatly  in  different  mammals.  It  develops  in  birds 
earlier  and  acquires  distention  first  in  the  future  cervical  region,  where  it  produces 
the  amnio-cardiac  vesicles  (Fig.  136,  A.c.v),  in  the  median  portion  of  whose 
united  cavities  the  heart  is  lodged.  The  splanchnocele  of  the  body  proper  appears 
after  the  primitive  segments,  and  its  expansion  takes  place  at  first  only  in  the  part 
of  the  mesoderm  next  to  tfre  primitive  segments.  Everywhere  as  the  splanchnocele 
develops  the  mesodermal  cells  about  it  assume  gradually  more  and  more  distinctly 
an  epithelial  character,  so  that  it  soon  becomes  proper  to  speak  of  the  mesothelium 
or  boundary  epithelial  wall  of  the  ccelom. 

The  splanchnocele  is  also  designated  by  several  other  names,  and  is  sometimes 
called  simply  the  body-cavity  or  somatic  cavity.  Others  term  it  the  ventral  ccelom. 
By  English  embryologists  it  is  usually  called  the  pleuro-peritoneal  space.  Its  future 
subdivisions  become  early  indicated  by  a  transverse  ridge  of  tissue  which  is  known 
as  the  septum  transversum.  This  septum  is  situated  at  the  posterior  end  of  the 
heart,  and  is  developed  to  allow  the  great  veins  to  have  access  to  the  heart  itself. 
It  is  the  anlage  of  the  future  diaphragm.  It  separates  the  ccelom  around  the  heart 
from  that  of  the  abdomen.  It  is  a  product  of  the  splanchnopleure,  so  that 
it  arises  upon  the  ventral  side  of  the  ccelom.  We  have,  as  soon  as  this  septum  is 
present,  the  pericardial  cavity  on  its  cephalic  side,  the  abdominal  cavity  on  its  cau- 
dal, and  a  small  pleural  cavity  on  its  dorsal  side. 

The  Ccelom  of  the  Head. — No  adequate  investigation  of  the  early  stages  of  the 
mesoderm  in  the  head  of  amniota  has  yet  been  made.  We  know,  however,  that 
in  the  lower  vertebrates  there  appear  at  least  three  distinct  cavities  resembling  por- 
tions of  the  true  ccelom  and  bounded  by  epithelial  cells,  similar  to  the  mesothelium 
in  character.  These  cavities  are  generally  regarded  as  portions  of  the  true  ccelom, 
and  by  many  writers  have  been  interpreted  as  true  primitive  segments.  But  this 
interpretation  is  not  yet  beyond  doubt.  The  largest  of  these  is  called  the  mandibu- 
lar  cavity,  because  it  has  a  prolongation  which  extends  into  the  mandible  of  the 
young  embryo.  In  front  of  it  is  the  first  or  premandibular  cavity,  which  is  much 
smaller,  and  behind  it  is  the  third  or  hyoid  cavity,  which  is  intermediate  in  size 
between  the  first  and  second  (Fig.  48).  The  head-cavities  are  best  known  in  the 
elasmobranchs.  They  have  also  been  found  clearly  developed  in  reptiles  and  cer- 
tain birds.  In  mammals  no  actual  cavities  have  been  recorded.  There  are  found 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


N.fac.ac.      Ot.        O.   N.gl.ph.       N.vag.     Dor.com. 


Hi. 


So.  ant 


Op.  ves. 


G.cl.     Md.st.       Hyo.st. 


Prcd. 


FIG.  48. — SQUALUS  ACANTHIAS,  9.0  MM.     SERIES  1495.     RECONSTRUCTION  TO  SHOW  THE  HEAD-CAVITIES. 

BY  R.  E.  SCAMMON. 

Ao,  Dorsal  aorta.  Dor.  com,  Dorsal  ganglionic  commissure.  G.cl,  First  gill-cleft;  two  others  are  open  and 
marked  by  similar  shading.  Ht,  Heart.  Hyo.  st,  Stalk  of  mesoderm  connecting  the  hyoid  cavity  with 
the  pericardium.  Md.  st,  Stalk  of  mesoderm  connecting  the  mandibular  cavity  with  the  pericardium. 
Nch,  Notochord.  N.  }ac.  ac,  Facial-acoustic  nerve  trunk.  2V.  gl.  ph,  Glossopharyngeal  nerve.  N.  vag, 
Vagus  nerve.  TV.  ar.  tr,  Temporary  ganglionic  mass  (Dohrn's  "Urtrochlearis").  O,  Orifice  of  the 
otocyst.  Op.ves,  Optic  vesicle.  Ot,  Otocyst.  Prcd,  Pericardium.  So.  ant,  Anterior  head-cavity,  a 
derivative  of  the  premandibular  cavity,  and  lacking  in  most  animals.  So.hyo,  Hyoid  head-cavity. 
So.mand,  Mandibular  head-cavity.  So.prem,  Premandibular  headcavity  X  30  diams. 


THE  MESENCHYMA. 


89 


the  anlages*  of  the  muscles  of  the  eye,  and  these  are,  by  hypothesis,  homologous 
with  the  cells  of  the  walls  of  the  head-cavities  in  the  lower  vertebrates,  which  cells 
produce  the  muscles  of  the  eye. 

The  Mesenchyma. 

By  the  term  mesenchyma  we  designate  the  whole  of  the  mesoderm  of  the  em- 
bryo, except  the  mesothelial  lining  of  the  coelom.  When  fully  differentiated  his- 
tologically,  it  consists  of  more  or  less  widely  separated  cells,  connected  with  one 
another  by  intervening  threads  of  protoplasm,  which  form  a  network  between  the 
cells  (Fig.  49).  The  remaining  space  is  filled  by  a  homogeneous  structureless  ma- 
trix or  basal  substance.  It  gives  rise  to  a  large 
number  of  adult  tissues,  as  shown  in  the  table 
on  page  19. 

In  the  early  development,  or  histogenesis,  of 
the  mesoderm  we  can  distinguish  four  stages: 
first,  that  of  distinct  cells;  second,  the  forma- 
tion of  the  cellular  network;  third,  the  forma- 
tion of  the  mesothelium;  and,  fourth,  the  differ- 
entiation of  the  mesenchyma.  The  first  stage  is 
known  chiefly  through  observations  on  the  early 
stages  of  elasmobranchs,  reptiles,  and  birds.  In 
these  types  the  first  cells  which  are  delaminated 
from  the  entoderm  to  form  the  anlage  of  the  meso- 
derm, are  of  quite  large  size  and  lie  between  the  entoderm,  or  yolk,  and  ectoderm, 
and  are  without  connection  with  one  another.  The  number  of  mesodermic  cells  in- 
creases both  by  the  multiplication  of  the  cells  already  delaminated  and  by  the  addition 
of  others  from  the  entoderm.  Whether  this  stage  occurs  in  mammals  or  not,  we  do 
not  know  at  present.  In  the  second  stage  the  primitive  cells  are  found  to  have  ac- 
quired connection  with  one  another,  the  protoplasm  of  one  cell  uniting  by  a  process, 
or  prolongation,  with  the  protoplasm  of  another  cell,  and  so  on  until  the  whole 
tissue  becomes  a  network.  When  the  primitive  streak  has  been  formed  in  the 
mammalian  blastodermic  vesicle  we  find  the  mesoderm  in  this  condition.  The  third 
stage  is  brought  about  by  the  development  of  the  coelom  as  above  described.  The 
coelom  is  bounded  by  the  undifferentiated  mesoderm.  To  produce  the  fourth  stage, 
single  cells  leave  the  primitive  mesodermic  layer  by  migrating  out  of  it  on  the 
side  away  from  the  ccelom.  The  cells  left  behind  are  ultimately  reduced  to  a  sin- 
gle layer,  the  permanent  mesothelium.  The  emigrant  cells  constitute  the  mesen- 
chyma and  are  found  to  be  connected  both  with  one  another  and  with  the  meso- 
thelial cells  by  protoplasmatic  processes,  but  they  do  not  lie  close  together,  as  in 
the  epithelium,  so  that  there  is  a  considerable,  though  variable,  amount  of  inter- 

*  The  anlages  may  be  seen  in  a  pig  embryo  of  10  mm.  between  the  jugular  vein  and  the  internal  carotid 
artery  as  a  group  of  embryonic  cells  quite  distinct  from  the  surrounding  mesenchyma. 


J8 
FIG.  49. — CHICK  EMBRYO  OF  THE  THIRD 

DAY. 

Mesenchyma  from  near  the  otocyst.     A,  cell 
in  mitosis. 


90  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

cellular  space.  By  the  migration  of  the  cells  and  their  multiplication,  the  mesen- 
chyma  is  produced.  It  fills  up  all  the  room  between  the  mesothelium  and  the  two 
primary  germ-layers  so  far  as  it  is  not  occupied  by  the  developing  blood-vessels  and, 
later,  by  lymph  vessels. 

Apparently  the  entire  mesothelium  may  participate  in  the  production  of  the 
mesenchymal  cells.  Its  different  regions,  however,  do  not  so  participate  all  to  an 
equal  degree,  or  at  the  same  time.  The  throwing  off  of  mesenchymal  cells  may  be 
observed  in  certain  parts  of  the  embryo  in  somewhat  advanced  stages  of  develop- 
ment, and  it  seems  not  impossible  that  the  process  may  be  found  to  occur  even 
in  adult  life. 

The  mesoderm,  by  the  formation  of  mesenchyma,  becomes  very  early  unlike 
the  other  germ-layers.  Both  ectoderm  and  entoderm  are  epithelial  membranes. 
The  mesoderm  is  partly  epithelial,  partly  mesenchymal,  and  from  the  mesenchyma 
arise  special  kinds  of  tissue  which  are  characteristic  of  the  middle  germ-layer,  and 
never  are  produced  from  either  the  outer  or  inner  germ-layers. 

The  Origin  of  the  Blood-vessels  and  Blood. 

As  stated  above  (pages  66  and  80),  the  angioblast  and  first  blood-vessels  ap- 
pear in  the  circumscribed  region  in  the  mesoderm  of  the  yolk-sac  and  lie  close 
against  the  entodermal  cells  of  the  area  opaca.  The  region  which  they  occupy  is 
termed  the  area  vasculosa.  From  the  area  vasculosa  the  development  of  blood- 
vessels extends,  as  stated,  across  the  area  pellucida  into  the  embryo.*  During  these 
early  stages  the  only  blood-vessels  are  in  the  splanchnopleure.  After  their  formation 
has  extended  into  the  body  of  the  embryo,  it  spreads  into  the  somatopleure  also, 
which,  therefore,  acquires  its  blood-vessels  at  a  later  stage.  It  should  be  noted, 
however,  that  the  development  of  the  blood-vessels  begins  before  the  ccelom  has 
been  developed  over  the  area  vasculosa.  While  they  are  forming,  the  ccelom 
expands;  and  after  it  has  appeared,  the  primitive  blood-vessels  are  found  always 
exclusively  in  the  splanchnic  mesoderm. 

Definition.— The  essential  part  of  a  blood-vessel  is  its  endothelial  wall.  In 
early  stages  all  the  blood-vessels  consist  only  of  endothelium.  Arteries  and  veins 
differ  but  little,  if  at  all,  in  histological  structure  during  early  embryonic  stages,  and 
are  distinguished  chiefly  by  the  direction  of  blood-currents  passing  through  them. 
Capillary  blood-vessels  and  sinusoids  have,  as  a  rule,  throughout  life  merely  an  en- 
dothelial wall.  Arteries  and  veins  become  strengthened  by  the  development  of 
special  coats  around  the  endothelium  which  arise  by  transformations  of  the  mesen- 
chymal cells  in  the  immediate  neighborhood  of  the  vessels. 

The  Development  in  the  Chick.— The  first  indication  of  the  blood-vessels  is  a  ret- 
iculate appearance,  which  can  be  recognized  in  the  mesoderm  in  surface  views  of 
the  fresh  or  hardened  embryo  at  the  end  of  the -first  day.  The  reticulate  structure 
increases  rapidly  in  extent  and  distinctness  during  the  second  day  of  incubation. 

*  It  has  been  recorded  that  in  lizards  the  vascular  anlages  appear  first  in  the  area  pellucida. 


THE  ORIGIN  OF  THE  BLOOD-VESSELS  AND  BLOOD. 


91 


It  is  confined  to  the  region  of  the  mesoderm  surrounding  the  embryo  proper,  and 
which  is,  therefore,  known  as  the  area  vasculosa,  as  above  stated  (compare  Fig.  131). 
As  soon  as  there  are  several  primitive  segments  in  the  embryo,  the  network  in  the 
mesoderm  shows  traces  of  coloration  in  irregularly  shaped  reddish  yellow  spots, 
which  are  largest  and  most  numerous  around  the  caudal  end  of  the  embryo.  These 
spots  are  called  blood-islands  (Fig.  131,  Bl.is)  because  the  central  cells  in  them 
are  transformed  into  the  first  blood-corpuscles.  The  network  appearance  is  due  to 
the  development  of  the  angioblast,  which  is  a  set  of  cells  delaminated  from  the  en- 
toderm  or  the  yolk,  and  intervening  between  the  mesoderm  proper  and  the  entoderm. 
The  angioblast  at  first  assumes  the  form  of  more  or  less  solid  cords.  The  meshes 


FIG.  50. — HUMAN  EMBRYO  OF  9.4  MM.    PROBABLE  AGE  THIRTY  DAYS.     X  8  diams. 

of  the  angioblast  are  partly  or  wholly  filled  by  mesodermic  cells.  The  ccelom  now 
appears  in  the  extra-embryonic  area,  and  thereafter  the  anlages  of  the  blood-vessels 
are  connected  with  the  splanchnic  mesoderm  only.  The  anlages  of  the  blood- 
vessel at  this  stage  form  a  thick  network  without  distinction  of  stem  or  branch,  ex- 
cept that  the  edge  of  the  area,  bounded  by  a  broad  band  of  angioblast,  gives  rise 
to  a  single  large  vessel,  which  is  known  as  the  sinus  terminalis.  The  anlages  are  all 
in  one  layer,  none  overlying  the  others,  and  up  to  this  stage  they  are  all  solid. 
The  terminal  sinus  becomes  connected  with  the  venous  system. 

The  blood-islands  are  spots  where  there  is  a  cluster  of  cells,  which  remain  at- 
tached to  one  another  and  to  the  walls  of  the  vessels.  The  cells  develop  hemo- 
globin in  their  interior,  hence  the  clusters  have  a  reddish  color  which  renders  all 
the  islands  very  conspicuous  in  surface  views  of  fresh  specimens.  Blood-islands  appear 
first  in  the  area  opaca,  but  almost  immediately  after  in  the  pellucida  also.  They 


92 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


have  at  first  a  rounded  or  branching  form.  In  the  inner  part  of  the  pellucida  they  are 
small  and  stand  alone.  Toward  the  periphery  they  are  larger,  more  closely  set, 
and  more  united  with  one  another.  Their  development  is  greater  around  the  caudal 
end  of  the  embryo. 

In  the  next  stage  the  vascular  anlages  become  hollow,  and  then  may  be 
called  true  blood-vessels.  When  they  acquire  a  lumen,  the  blood-islands  are  found 
to  remain  attached  usually  to  the  upper  side  of  the  vessel  like  a  thickening  of  its 
wall  (Fig.  51,  bl.is).  Very  soon  after  the  vessels  have  become  hollow  the  cells  of 
the  blood-islands  break  apart  and  lie  free  in  the  cavity  of  the  vessel,  thus  forming 
the  first  blood-corpuscles.  They  are  characterized  by  having  a  rounded  nucleus 
with  a  very  distinct  nucleolus,  and  a  minimal  covering  of  protoplasm  only.  After 


som 


FIG.  51. — SECTION  OF  THE  AREA  VASCULOSA  OF  A  CHICK  EMBRYO  OF  THE  SECOND  DAY. 

Som,  Somatopleure.     Spl,  Splanchnopleure.     EC,  Ectoderm.     En,  Entoderm.     bl.is,  Blood-isands.     V,  V,  Blood- 
vessels.     X  227  diams. 

the  cells  have  become  free  the  amount  of  protoplasm  in  each  cell  increases.  The 
cells  multiply  rapidly  by  mitotic  division.  It  is  believed  that  all  the  blood- 
corpuscles,  both  red  and  white,  are  descendants  of  these  cells  derived  from  the 
blood-islands. 

The  angioblast  continues  growing  by  the  development  of  buds  from  the  vessels 
already  formed.  These  buds  are  rounded  or  pointed,  forming,  as  it  were,  spurs. 
They  often  end  by  meeting  one  another  and  uniting.  They  are  usually  hollow  from 
the  first,  and  after  they  meet  one  another  or  an  adjacent  vessel,  the  cavities  be- 
come continuous,  and  thus  the  vascular  network  is  extended. 

The  Development  in  Mammals. — The  origin  of  the  blood-vessels  in  mammals  is 
not  adequately  known.  The  solid  primary  anlages  appear  in  the  extra-embryonic 
area  vasculosa  and  extend  later  into  the  embryo.  They  present  well-marked  blood- 
islands,  which  make  their  first  appearance  in  rabbit  embryos  of  the  eighth  day, 
just  before  the  appearance  of  the  first  primitive  segments.  It  is  characteristic  of 
most  mammals  that  the  entire  yolk-sac,  probably  owing  to  its  small  size,  becomes, 
very  early  indeed,  vascularized  throughout. 


THE  BLOOD-CORPUSCLES.  93 

The  Growth  of  the  Vessels  into  the  Embryo. — The  entrance  of  the  vessels  into 
the  embryo  chick  begins  toward  the  end  of  the  second  day.  The  buds  which  form 
the  extra-embryonic  angioblast  grow  first  toward,  then  into,  the  embryo.  The  pene- 
trating vessels  follow  certain  prescribed  paths.  Part  of  the  vessels  run  along  the 
posterior  edge  of  the  amnio-cardiac  vesicles,  and  enter  into  connection  with  the  pos- 
terior end  of  the  heart,  which  has  meanwhile  been  progressing,  and  which — owing 
to  the  early  separation  of  the  head  end  of  the  embryo  from  the  yolk — is  the  only 
part  of  the  heart  which  the  vessels  can  reach  directly.  While  the  vessels  are  ap- 
proaching the  heart  their  differentiation  into  various  sizes  is  going  on,  the  smallest 
ones  to  remain  as  capillaries,  the  larger  ones  to  become  arteries  or  veins.  The 
only  two  veins  in  the  first  stage  are  those  above  mentioned,  which  are  called  the 
omphalo-mesaraic.  Another  set  of  vessels  penetrates  along  the  splanchnopleure  of 
the  body  on  each  side  until  they  attain  the  small  space  between  the  notochord  and 
somite  and  the  entoderm,  where  they  fuse  so  as  to  form  a  longitudinal  vessel,  the 
anlage  of  the  descending  aorta  (Fig.  143,  Ao.D).  It  should  be  noted  that  this 
anlage  is  primitively  double.  The  aorta  appears  first  in  the  region  toward  the 
head.  It  grows  forward  above  the  pharynx,  bends  ventrally  just  behind  the  mouth, 
dividing  as  it  bends,  one  branch  going  around  each  side  of  the  future  pharynx,  and 
uniting  again  on  the  ventral  side  of  the  pharynx  in  the  mediari  ventral  line,  in 
order  to  join  the  anterior  end  of  the  tubular  heart.  The  heart  begins  to  beat  be- 
fore the  vessels  unite  with  it.  The  first  blood-cells  have  already  been  formed; 
hence  as  soon  as  union  is  accomplished  the  blood  circulation  starts  up,  the  blood 
passing  through  the  aorta  to  the  body,  thence  by  numerous  lateral  branches  to  the 
area  vasculosa,  and  returning  by  the  two  omphalo-mesaraic  veins  to  the  heart.  It 
will  thus  be  seen  that  almost  the  entire  circulation  is  extra-embryonic. 

The  other  embryonic  blood-vessels  are  developed  by  buds  from  the  walls  of 
the  vessels  already  present  in  the  embryo,  in  the  same  general  manner  as  new  ves- 
sels are  formed  in  the  area  vasculosa.  These  buds  give  rise  to  the  endothelium 
only  of  the  embryonic  vessels.  When  a  vessel  becomes  an  artery  or  a  vein,  the 
media  and  adventitia  are  added,  as  above  stated,  by  differentiation  of  the 
surrounding  mesenchyma. 

During  further  development  many  small  blood-vessels  abort,  and  often  appear 
as  disconnected  bits,  closed  at  both  ends  and  containing  corpuscles.  Such  struc- 
tures were  at  one  time  supposed  to  be  developing  blood-vessels  and  were  accord- 
ingly termed  "  vaso-f ormative  cells."  The  blood-corpuscles  in  them  are  of  course 
not  developing,  but  degenerating  . 

The  Blood-corpuscles. 

The  first  blood-corpuscles  are  free  cells  of  an  indifferent  character  and  capable 
of  wandering  through  the  walls  of  the  blood-vessels,  which  in  early  stages  are  easily 
permeable,  as  if  of  a  merely  gelatinous  consistency.  The  primitive  blood-cells,  as 
they  may  be  called,  give  rise  not  only  to  the  permanent  blood-corpuscles,  both  red 


94  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

and  white,  but  various  other  cells  outside  of  the  vessels,  of  which  two  classes  are 
especially  important — the  free  wandering  cells  in  the  mesenchyma  or  connective 
tissue  and  the  giant  cells.  The  latter,  however,  contribute  to  the  blood,  for  they 
form  in  the  spleen,  bone-marrow,  and  other  organs,  long  processes,  like  pseudo- 
podia,  which  break  up  into  fragments.  These  fragments  are  the  blood-plates.  It 
is  probable  that  all  blood-cells  and  wandering  cells  are  exclusively  descendents 
of  the  primitive  blood-cells,  although  some  writers  maintain  that  their  number 
is  increased  in  both  the  embryo  and  the  adult  by  transformation  of  cells  of  the 
mesenchyma. 

When  the  circulation  begins,  the  number  of  corpuscles  is  small,  but  it  rapidly 
increases  by  mitotic  division  of  the  cells.  At  the  very  start,  like  all  cells  produced 
by  segmentation  of  the  ovum,  the  blood-cells  are  quite  large,  but  they  rapidly  de- 
crease in  size  until  they  reach  the  "first-stage,"  in  which  they  appear  as  small 
round  cells  (in  the  chick  8.3  to  I2.5//  in  diameter)  with  a  rounded  granular  reticu- 
late nucleus  and  a  minimal  amount  of  protoplasm.  In  the  next  stage  the  amount 
of  protoplasm  increases.  We  have  next  to  consider  separately  the  cytomorphoses  of 
the  red  and  white  corpuscles. 

Red  Corpuscles. — By  examining  the  blood  of  chick  embryos  of  successive  ages 
we  can  trace  the- differentiation  of  the  red  cells.  We  find  that  the  protoplasm 
enlarges  for  several  days,  and  that  during  the  same  time  there  is  a  progressive 
diminution  in  the  size  of  the  nucleus,  which,  however,  is  completed  before  the  area 
of  protoplasm  reaches  its  ultimate  size.  The  nucleus  is  at  first  granular,  and  its 
nucleolus  or  nucleoli  stand  out  clearly.  As  the  nucleus  shrinks,  it  becomes  round 
and  is  colored  darkly,  and  almost  uniformly,  by  the  usual  nuclear  stains.  This 
change  is  called  pyknosis.  The  blood-cells  of  mammals  pass  through  the  same 
metamorphosis  as  those  of  birds.  For  example,  in  rabbit  embryos  of  eight  days 
(Fig.  52,  A)  the  cells  have  reached  the  stage  with  a  granular  nucleus  and  well- 
developed  cell-body.  Corpuscles  of  this  kind  are  characteristic  of  fishes  and 
amphibia,  and  they  may,  therefore,  be  designated  as  the  ichthyoid  cells.  Two  days 
later  the  nucleus  is  already  smaller,  and  by  the  thirteenth  day  has  shrunk  to  its 
final,  dimensions.  The  cells  in  this  condition  are  characteristic  of  the  reptiles  and 
birds,  and  may  be  designated,  therefore,  as  sauroid  cells.  The  nucleated  stage  of 
the  cells  is  typical  of  embryonic  life  only  in  mammals.  During  the  fetal  period 
the  nuclei  of  the  red  cells  gradually  disappear  and  the  cells  are  transformed  into 
the  non-nucleated  corpuscles,  which  occur  only  in  mammals,  so  that  this  last  may 
be  designated  as  the  mammalian  stage.  The  nuclei  disappear  by  extrusion  from 
the  cells.  Usually  they  break  into  fragments,  which  are  then  expelled.  Some- 
times, though  rarely,  the  nucleus  goes  out  intact.  The  successive  stages  of  the 
blood-corpuscles  in  mammals  illustrate  the  law  of  recapitulation  (page  29).  When 
the  nucleus  disappears,  the  corpuscle  becomes  smaller.  In  the  human  embryo  at 
one  month  the  red  cells  are  the  predominant  blood-corpuscles.  At  two  months 
they  are  still  the  most  numerous,  although  the  non-nucleated  corpuscles  have  begun 


THE  BLOOD-CORPUSCLES. 


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96  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

to  appear.  At  three  months  the  non-nucleated  corpuscles  constitute  by  far  the 
majority  of  all  corpuscles  in  the  blood. 

Leucocytes. — The  primitive  blood-cells,  being  colorless,  have  been  termed  leuco- 
cytes by  some  authors,  but  they  are  obviously  different  from  the  leucocytes  of  the 
adult  blood.  Some  of  them  become  so-called  lymphocytes  (young  leucocytes), 
which  are  distinguishable  from  the  primitive  cells  by  the  internal  structure  of  the 
nucleus.  Others  grow  in  size  and  follow  at  least  two  cytomorphic  paths.  In  one 
series  the  protoplasm  develops  fine  granules,  and  the  nucleus  becomes  first  elongate, 
then  reniform,  and  finally  beaded.  In  this  form  they  appear  as  polymorpho- 
nuclear  neutrophile  leucocytes.  In  the  second  series  the  protoplasm  acquires  coarser 
granules,  which  are  really  phagocyted  morsels  of  red  blood-cells,  and  the  nucleus 
becomes  reniform.  These  cells  are  termed  the  eosinophile  leucocytes. 

The  term  leucocyte  properly  embraces  all  the  white  corpuscles  of  the  adult 
blood,  but  has  been  erroneously  restricted  by  some  recent  authors  to  the  granular 
forms.  The  young  stages  of  the  granular  leucocytes  are  sometimes  termed  myelo- 
cytes,  because  in  the  adult  they  occur  chiefly  in  the  bone-marrow,  which  is  the 
chief  sanguifactive  tissue  in  the  adult. 

It  is  doubtful  whether  leucocytes  ever  develop  in  the  circulating  blood.  They 
appear  abundantly  after  the  lymph-glands  are  formed.  The  usual  explanation  is 
that  some  of  the  wandering  primitive  blood-cells  enter  the  glands,  there  multiply, 
and  in  part  become  leucocytes. 

The  Origin  of  the  Heart. 

The  manner  in  which  the  head  of  the  embryo  becomes  free  is  described  on  page 
49  (compare  also  Figs.  16  and  132).  The  origin  of  the  ccelom  is  described  on  page 
8 1.  When  the  head  becomes  free,  the  ccelom  is  found  soon  to  extend  across  the 
median  line.  This  takes  place  at  the  cervical  end  of  the  head  just  where  the 
tissues  of  the  embryo  bend  over  to  join  the  yolk  (Fig.  132,  p.  183).  This  median 
ccelom  is  the  beginning  of  the  pericardial  cavity.  In  connection  with  it  the 
development  of  the  heart  occurs.  The  formation  of  this  organ  is  probably  initiated 
by  an  ingrowth  of  the  cells  of  the  angioblast,  which  give  rise  to  the  endothelium 
of  the  heart  (Fig.  138,  Endo).  The  mesothelium  of  the  dorsal  side  of  the  primitive 
pericardial  coelom  produces  the  muscular  walls  of  the  heart  (Fig.  129,  m.ht).  The 
early  development  and  primitive  relations  of  this  organ  can  be  understood  by  the 
account  given  in  Chapter  V  of  the  structure  of  a  chick  embryo  with  eight 
segments. 

The  Germinal  Area. 

The  germinal  area  is  that  portion  of  the  amniote  ovum  (mammalian  blastodermic 
vesicle)  in  the  center  of  which  the  embryo  is  differentiated.  It  comprises,  therefore, 
both  the  embryo  proper  and  the  region  immediately  surrounding  it.  In  mammals 
it  corresponds  in  extent  with  the  embryonic  shield  (p.  47).  In  its  center  we  find 


THE  MAIN  VESSELS  OF  THE  AREA   VASCULOSA.  97 

the  anlages  of  the  embryonic  structures  proper.  In  its  extra-embryonic  part  we 
find  the  three  primitive  germ-layers.  Underneath  the  entoderm  is  the  cavity  of  the 
yolk-sac.  In  the  mesoderm  we  have  occurring  the  development  of  the  ccelora,  and 
in  the  splanchnic  mesoderm  the  differentiation  of  the  primitive  blood-vessels.  These 
primitive  vessels  occupy  the  sharply  denned  territory,  the  edge  of  which  is  marked 
by  the  sinus  terminalis  (Fig.  131,  V.t}.  The  first  differentiation  in  the  germinal 
area,  which  can  be  clearly  recognized  by  the  naked  eye,  is  the  appearance  of  the 
area  pellucida,  which  is  due  to  the  thinning  of  the  entoderm  over  the  central 
area.  Next  ensues  the  differentiation  of  the  primitive  streak  (Fig.  14).  Further 
progress  results  in  the  gradual  differentiation  of  the  embryo,  in  the  sharp  demarca- 
tion of  the  area  pellucida,  which  becomes  pear-shaped,  and  in  the  appearance  of 
the  blood-vessels  and  the  resulting  differentiation  of  the  area  vasculosa  or  opaca. 
Figure  131,  on  page  182,  represents  the  embryonic  area  of  a  hen's  ovum  after  about 
twenty-seven  hours'  incubation.  The  embryo  is  well  advanced  in  development,  for, 
although  the  primitive  streak,  pr.s,  still  remains  in  part  and  the  medullary  groove  is 
still  open  behind,  the  brain  is  already  marked  out  and  the  head  has  become  partly 
free.  Alongside  the  medullary  canal  lie  eight  pairs  of  segments.  Around  the  em- 
bryo one  easily  recognizes  the  somewhat  pear-shaped  area  pellucida,  A.p,  and  the 
darker  area  opaca,  by  which  it  is  enclosed.  The  area  vasculosa  stands  out 
conspicuously  and  is  bounded  by  the  already  distinguishable  sinus  terminalis  V.t. 
Around  and  underneath  is  the  translucent  pro-amnion,  pro.am,  from  which  the 
mesoderm  is  altogether  absent,  and  which,  therefore,  cannot  contain  any  blood- 
vessels. Nor  are  there  at  this  state  any  vessels  in  front  of  the  pro-amnion.  The 
general  topographical  arrangement  is  the  same  in  mammals  (compare  page  179 
and  Fig.  130). 

The  Main  Vessels  of  the  Area  Vasculosa. 

Soon  after  the  capillary  network  of  the  areas  opaca  and  pellucida  has  penetrated 
the  embryo,  certain  lines  of  the  network  begin  to  widen,  and  soon  distinctly  assume 
the  size  and  functions  of  main  trunks;  some  of  these  unite  with  the  posterior  venous 
end  of  the  heart  (Fig.  59,  Ve.ht),  which  has  meanwhile  been  formed  in  the  em- 
bryo, and  others  become  connected  with  the  anterior  or  aortic  end.  Even  before  this 
the  heart  has  begun  to  beat,  so  that,  as  soon  as  all  connections  are  made,  the  primi- 
tive circulation  starts  up.  The  arrangement  of  the  vessels  is  not  the  same  in  birds 
and  mammals.  The  disposition  in  birds  is  indicated  by  the  diagram  shown  in 
figure  53,  in  which,  it  should  be  remembered,  the  embryo  and  the  capillary  net- 
work are  drawn  many  times  too  large  in  proportion  to  the  area  vasculosa.  The 
area  is  bounded  by  a  broad  circular  vessel,  the  sinus  terminalis,  S.T,  which  con- 
stitutes a  portion  of  the  venous  system  in  birds,  for  in  front  of  the  head  of  the 
embryo  the  sinus  leaves  a  gap,  and  is  reflected  back  along  the  sides  of  the  body  of 
the  embryo  to  make  two  large  veins,  which,  after  uniting  with  the  other  venous 
channels  coming  from  various  parts  of  the  area  vasculosa  on  each  side,  enter  the 

7 


98 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


embryo  as  two  large  trunks,  Om.V,  known  as  the  omphalo-mesaraic  veins;  these 
two  veins  unite  in  a  median  vessel,  the  sinus  venbsus,  S.V,  which  runs  straight 
forward  and  enters  the  posterior  end  of  the  heart.  The  sinus  venosus  also  receives 
the  veins  from  the  body  of  the  embryo,  namely,  the  anterior  cardinals,  Jug,  and 
posterior  cardinals,  card.  The  two  cardinals  of  each  side  unite,  making  a  short 
transverse  trunk  known  as  the  common  cardinal,  D.C,  which  in  turn  empties  into 
the  sinus  venosus.  The  entire  venous  current  is  thus  brought  to  the  heart  in  a 
united  stream;  it  passes  out  through  the  aorta,  the  greater  part  ascends  the  aortic 


P.O. 


Ora./L 


FIG.  53. — DIAGRAM  OF  THE  CIRCULATION  IN  A  CHICK  AT  THE  END  OF  THE  THIRD  DAY,  AS  SEEN  FROM  THE  UNDER 

(ENTODERMAL)  SIDE. 
The  embryo,  with  the  exception  of  the  heart,  is  dotted;  the  veins  are  black.     Ao,  Aorta.     Arc,  Aortic  arches. 

card,   Posterior  cardinal   vein.     D.C,   Common   cardinal   vein.     Ht,   Heart.    Jug,   Anterior  cardinal   vein. 

Om.A,  Omphalo-mesaraic  or  vitelline    artery.     Om.V,  Omphalo-mesaraic   or    vitelline   vein.    S.T,  Sinus 

terminalis.    S.V,  Sinus  venosus. 

arches  and  passes  back  through  the  main  aorta,  Ao,  and  divides  at  the  posterior 
fork  of  the  aorta,  the  bulk  of  the  two  currents  passing  out  through  omphalic"  ar- 
teries, Om.A,  and  thence  to  the  capillaries  of  the  area  vasculosa  and  so  on  to  the 
venous  trunks  again.  As  shown  in  the  figure,  which  presents  the  under  side  of  the 
area,  the  left  omphalo-mesaraic  vein  preponderates,  and  in  the  latter  stages  this  dif- 
ference becomes  more  marked,  until  finally  the  right  stem  is  very  inconsiderable  in 
comparison  with  the  great  left  vein.  The  time  at  which  the  disparity  commences 
is  extremely  variable,  as  is  also  the  degree  of  inequality  between  the  two  veins. 

The  following  description  probably  represents  what  was  the  primitive  condition 
of  vessels  in  the  mammalian  area  vasculosa.  It  applies  to  an  early  stage  in  the 
rabbit.  An  essentially  similar  arrangement  of  the  vessels  exists  also  at  a  correspond- 


THE  AORTIC  SYSTEM. 


99 


ing  stage  in  the  dog.  The  veins  are  much  more  symmetrical  than  in  the  chick,  and 
have  the  same  general  plan;  the  sinus  terminalis  belongs  to  the  venous  system,  so 
that  the  connection  with  the  arterial  circulation,  found  later,  is  secondary;  the  aorta 
of  the  embryo  is  double,  and  gives  off  on  each  side  (segmentally  arranged?)  trans- 
verse branches,  one  of  which  develops  into  the  large  trunk  shown  in  figure  54;  the 
network  of  small  vessels  forms  two  layers,  of  which  the  upper  is  connected  with 
the  arteries,  the  lower  with  the  veins.  The  change  from  the  earlier  condition  to 
the  later  has  still  to  be  followed. 


FIG.  54.— AREA  VASCULOSA  OF  A  RABBI?,  PRESUMABLY  OF  ABOUT  TWELVE  DAYS. — (After  Van  Beneden  and  Julin.) 

The  arrangement  of  the  main  vessels  in  the  area  vasculosa  at  a  later  stage 
in  the  rabbit  is  quite  different.  The  sinus  terminalis  forms  a  complete  ring  (Fig. 
54),  and  is  connected  with  the  arterial  system  by  a  single  trunk,  which  corresponds 
to  the  left  omphalic  artery  of  the  bird.  For  some  time  the  connection  between  the 
embryonic  arteries  and  the  area  vasculosa  is  entirely  through  capillaries,  and  the 
arterial  trunk  on  the  vascular  area  does  not  appear  in  the  rabbit  for  several  days. 
There  are  two  veins,  one  arising  from  each  side  of  the  body  and  passing  out  on 
to  the  area  vasculosa  over  the  back  of  the  embryo;  they  are  the  two  large  upper 
vessels  in  the  figure. 

The  Aortic  System. 

In  early  stages  the  aortic  end  of  the  heart  terminates  under  the  ventral  floor 
of  the  pharynx.  The  endothelial  heart  continues  as  the  aorta,  which  almost  at 


100 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


once  branches  to  the  right  and  left.  Each  branch  forms  five  vessels,  the  so-called 
aortic  arches  (Figs.  55,  92,  and  93).  The  first  arch  is  the  first  formed,  the  other 
four  are  formed  in  succession  behind  it.  The  arches  show  a  constant  relation  to 
the  pharyngeal  gill-pouches,  there  being  one  pouch  between  every  two  arches.  The 
arches  pass  dorsalward  around  the  broad  pharynx  (Fig.  28),  and  those  of  each 
side  become  united  by  a  single  dorsal  longitudinal  vessel  (Fig.  56,  Ao.D},  the 
dorsal  or  descending  aorta.  The  two  dorsal  aortae  pass  caudad  until  they  meet 
and  unite  in  the  median  line  to  form  the  main  aorta..  (Compare  Figs.  196,  Ao.S; 


Ao.D 


.  IV 


II 


XEO 


Coe. 


Ao.M 


Car.  ex 


FIG.  55. — ANTERIOR  WALL  OF  THE  PHAR-    FIG.  56. — PIG  EMBRYO  OF  6.0  MM.     SERIES  9.     AORTIC  ARCHES  OF 

YNX  OF  A  HUMAN  EMBRYO  OF  3 . 2  MM.  LEFT  SIDE.     FROM  A  WAX  RECONSTRUCTION  BY  L.  M.  FERGUSON. 

1-5,  Gill-arches;  the  arches  are  separated     I,  II,  III,  TV,  V,  Aortic  arches.  Car. ex,  External  carotid.   Ao,  Cardiac 

from  one  another  by  the  entodermal  aorta.     Ao.  D,  Dorsal  aorta.     Ao.M  Median  aorta. 

and  thecorrespondingectodermal  gill- 
pouches;  the  aortic  arches  are  drawn 

in  dotted  lines  and   arise   from  the 

median    cardiac  aorta.     M,  Mouth. 

Oe,  (Esophagus.  Cce,  Coelom.     X  50 

diams.— (After  W.  His.) 

197,  Ao,  and  198,^0.)  In  a  pig  of  7.8  mm.  the  five  aortic  arches  can  be  still 
traced,  but  the  first  arch  has  begun  to  disappear,  and  the  condition  illustrated  in 
figure  169  is  established.  Its  ventral  portion,  /,  persists,  however,  and  together  with 
its  own  vascular  prolongations  into  the  lower  Jaw  gives  rise  to  the  external 
carotid  (Fig.  101,  car. ex).  The  descending  aorta  on  the  dorsal  side  between  the 
tops  of  the  first  and  second  arches  also  persists  and  is  prolonged  into  the  head  to 
constitute  the  internal  carotid  (Fig.  172,  car.i).  Presently  the  second  arch  also 
disappears,  and  both  carotids  are,  as  it  were,  thereby  lengthened.  This  is  the 
condition  which  we  find  in  our  embryo  of  12  mm.  (Fig.  172).  The  third,  fourth, 
and  fifth  arches  are  still  present.  From  the  base  of  the  third  arch  runs  forward 
the  external  carotid,  and  from  the  summit  of  the  third  arch  runs  forward  the 


THE  AORTIC  SYSTEM.  101 

internal  carotid.  The  dorsal  ends  of  the  third  and  fourth  arches  are  still  con- 
nected, but  this  connection,  instead  of  being  a  large  aortic  vessel,  as  in  earlier 
stages,  has  now  contracted  and  almost  disappeared,  and  will  soon  be  lost  altogether, 
so  that  in  the  adult  there  will  be  no  connection  between  the  dorsal  ends  of  the 
third  and  fourth  arches.  The  fifth  arch  is  still  connected  with  the  dorsal  end  of 
the  fourth.  It  gives  off  the  small  pulmonary  artery  to  the  lungs.  On  the  side 
toward  the  heart  the  relations  of  the  arches  are  also  changed.  The  main  aortic 
vessel  which  springs  from  the  heart  is,  in  the  12  mm.  pig,  divided  into  two  vessels 
— the  pulmonary  aorta  on  the  ventral  side  and  the  true  aorta  in  a  more  dorsal 
position.  The  division  has  'so  taken  place  that  the  third  and  fourth  arches  are 
connected  only  with  the  true  aorta,  while  the  fifth  arch  is  connected  only  with  the 
pulmonary  aorta.  The  part  of  the  fifth  arch  on  the  left  side  between  the  origin 
of  the  pulmonary  artery  proper  and  the  main  descending  aorta  offers  at  this  stage 
an  open  communication  between  the  pathways  of  the  pulmonary  and  of  the  main 
body  circulation.  This  dorsal  half  of  the  fifth  aortic  arch  is  known  as  the  ductus 
arteriosus.  It  remains  throughout  the  fetal  period  as  an  open  channel,  so  that  the 
blood  from  the  right  ventricle  flows  in  part  to  the  lungs,  in  part  into  the  dorsal 
aorta.  The  lumen  of  the  ductus  arteriosus  disappears  in  man  soon  after  birth. 
As  an  anomaly  it  occasionally  persists  throughout  life,  involving  serious  modifica- 
tions of  the  normal  circulation.  The  dorsal  part  of  the  fifth  aortic  arch  of  the 
right  side  has  a  different  history,  for  it  aborts  early  in  embryonic  life,  and  there 
also  occurs  an  abortion  of  the  entire  descending  aorta  from  the  end  of  the  fourth 
arch  on  the  right  side  downward  to  the  level  of  the  diaphragm.  When  this  abor- 
tion has  taken  place,  the  entire  aortic  stream  flows  from  the  heart  to  the  left  side 
of  the  embryo.  The  aortic  branches  on  the  right  side  appear  as  follows  in  the 
adult:  The  main  stem,  from  which  the  five  arches  originally  sprang,  is  the  arteria 
innominata,  which  gives  off  a  stem,  the  common  carotid,  from  which  spring  the 
two  carotids  of  the  right  side.  Next,  a  vessel  which  represents  the  persistent  fourth 
right  arch,  which  no  longer  has  any  direct  communication  with  the  aorta,  but  at 
its  end  gives  off  the  subclavian  and  vertebral  arteries.  The  vessel  which  corre- 
sponds to  the  right  fourth  arch  is  usually  described  as  a  portion  of  the  stem  of  the 
subclavian  artery  in  the  adult.  The  aortic  branches  on  the  left  side  appeal,  as 
follows  in  the  adult :  first,  the  common  carotid,  the  stem  of  the  original  first  to 
third  arches;  second,  the  subclavian,  which  includes  a  part  of  the  fourth  arch,  but 
consists  chiefly  of  what  was  originally  only  a  branch  of  the  arch.  The  pulmonary 
artery  has  as  its  stem  a  portion  of  the  fifth  arch,  in  man  on  the  left  side  only, 
but  the  vessel  arises  as  a  separate  branch  from  the  fifth  arch. 

There  often  appear  irregular  vessels  in  early  stages  between  the  fourth  and 
fifth  arches,  and  these  have  been  held  by  some  writers  to  represent  an  additional 
partially  aborted  aortic  arch.  If  this  is  the  case,  then  the  arch  here  called  the 
fifth  would  be  more  correctly  termed  the  sixth.  The  arch  supposed  to  be  lost  is 
sometimes  distinguished  as  Zimmermann 's  arch. 


102 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


The  descending  and  median  aortae  give  off  intersegmental  vessels,  some  of 
which  persist  in  the  adult.  The  main  aorta  produces  three  main  branches  on  its 
ventral  side,  the  histories  of  which  are  somewhat  complicated.  They  are  the 
coeliac  axis,  the  superior  mesenteric,  and  the  inferior  mesenteric. 

The  Venous  System. 

The  veins  do  not  for  the  most  part,  if  at  all,  arise  as  independent  vessels,  but 
by  the  transformation  of  channels  in  a  network  of  small  vessels.  Arteries  develop 
in  the  same  way,  but  with  them  the  growth  of  the  main  stem  as  such  plays  a 


FIG.  57. — CHICK  EMBRYO  WITH  SEVENTEEN  SEGMENTS.     DRAWN  FROM  A  SPECIMEN  WHICH  HAD  BEEN  AUTO- 
INJECTED  WITH  INDIA-INK. — (After  H.  M.  Evans.) 


greater  role.  The  method  of  development  is  illustrated  by  figures  57  and  58,  From 
the  first  aortic  arch  a  network  of  small  vessels  spreads  into  the  head  at  the  sides 
of  the  fore-brain  and  mid-brain,  gradually  occupying  an  increasing  territory-  until  the 
whole  head  is  supplied.  The  plexus  forms  a  single  vascular  layer  between  the  brain 
and  epidermis  and  is  at  first  without  any  main  channels  (Fig.  57).  Soon  some  of 
the  capillaries  enlarge,  forming  a  branching  system  (Fig.  58),  the  branches  leading 
into  a  main  stem  which  extends  from  the  head  to  the  posterior  or  venous  end  of 
the  heart.  This  stem,  which  rapidly  increases  in  size,  is  the  anterior  cardinal  vein, 


THE  VENOUS  SYSTEM. 


103 


which  is  matched  by  a  similar  vein  on  the  opposite  side  of  the  head.  The 'remain- 
ing primitive  veins  and  the  later  secondary  veins  are  all  formed  in  a  like  manner. 
The  first  veins  to  be  formed  are  the  omphalo-mesaraic,  which  are  evolved  from 
the  vascular  network  of  the  area  vasculosa,  and  extend  into  the  body  of  the  em- 
bryo, running  in  the  splanchnopleure  (Fig.  158,  Om.S,  Om.D).  They  approach 
one  another  in  the  median  line  (Fig.  59),  unite,  and  are  prolonged  forward  to  make 
the  endothelial  heart. 


FIG.  58. — CHICK  EMBRYO  WITH  TWENTY-FIVE  SEGMENTS.     DRAWN  FROM  A  SPECIMEN  WHICH  HAD  BEEX  AUTO- 
INJECTED  WITH  INDIA-INK. — (After  H.  M.  Evans.) 


There  are  three  other  pairs  of  main  primitive  veins,  all  developed  entirely  within 
the  body  of  the  embryo:  i,  the  anterior  cardinals,  which  drain  the  head  (com- 
pare Fig.  154,  card,  and  Fig.  155,  Ve)\  2,  the  posterior  cardinals,  which  drain  the 
body  from  the  tail  to  the  heart  and  occupy  each  a  characteristic  position  laterad 
from  the  aorta,  and  dorsad  from  the  splanchnocele  (Fig.  158,  card,  and  Fig.  159, 
card,  card.s};  the  two  cardinal  veins  unite  at  the  level  of  the  venous  end  of  the 
heart  and  form  thus  on  each  side  a  short  transverse  stem,  the  common  cardinal, 
which  opens  into  the  heart  and  was.  formerly  named  the  ductus  Cuvieri;  3,  the 


104 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


umbilical  veins,  which  are  differentiated  a  little  later  from  the  vascular  plexus  of 
the  somatopleure.  (Compare  Figs.  90,  93,  and  the  description  of  the  veins  in  the 
human  embryo  of  the  ninth  stage,  p.  143.) 

The  pulmonary  veins  represent  a  new  system  of  vessels  added  to  those  already 
mentioned.  Their  exact  origin  has  not  been  traced,  but  about  the  time  the  branch- 
ing of  the  entodermal  lung  begins,  they  appear  and  open  into  the  left  auricle  of 
the  heart. 

The  eight  pairs  of  primitive  veins  pass  through  complicated  metamorphoses 
owing  principally,  first,  to  the  development  of  new  branches  to  receive  the  blood 
from  the  organs  as  they  arise;  second,  to  the  conversion  of  small  collateral  channels 


Ao.ht. 


Ve.ht. 


Om.mes. 


FIG.  59. — CHICK  OF  ABOUT  40  HOURS.    VIEW  PROM  UNDERNEATH  OF  PART  OF  THE  VASCULAR  SYSTEM. 

Ao.ht,  Aortic  limb  of  endothelial  heart.     Om.mes,  Omphalo-mesaraic  vein.     PI,  Part  of  the  vascular  plexus  of  the 

area  vasculosa.     Ve.ht,    Venous  end  of  the  endothelial  heart.      X  25  diams. 

into  main  vessels;  and,  third,  to  the  obliteration  of  parts  of  the  original  vessels. 
The  first  process  is  illustrated  by  the  history  of  the  veins  of  the  limbs;  the  second 
by  the  formation  of  the  lateral  vein  of  the  head  and  of  the  subcardinal  vein  of 
the  Wolffian  body;  the  third,  by  the  disappearance  of  the  right  umbilical  vein  and 
of  portions  of  the  omphalo-mesaraic  veins. 

The  vena  cava  inferior  is  a  new  pathway  formed  by  utilizing  portions  of  several 
originally  distinct  and  separated  venous  channels.  At  first  the  blood  from  the  ab- 
dominal viscera  must  return  to  the  heart  chiefly  through  the  posterior  cardinal 
veins  (Fig.  53,  card),  but  the  vena  cava  inferior  offers  a  direct  course.  Its 
development  depends  primarily  upon  the  union  of  the  cephalad  end  of  the  right 
Wolffian  body  with  the  liver,  followed  by  a  vascular  fusion  of  the  two  organs  which 
renders  it  possible  for  the  blood  of  the  right  subcardinal  vein  to  pass  through  the 


THE  LYMPHATIC  SYSTEM.  105 

blood-spaces  of  the  liver  directly  to  the  heart.  This  makes  a  very  direct  channel,  a 
more  direct  one  than  existed  previously  when  the  blood  from  the  subcardinal  came  to 
join  that  of  the  cardinal,  passing  up  to  the  common  cardinal  and  then  back  to  the 
heart.  The  new  channel  through  the  liver  rapidly  enlarges  and  becomes  recogniz- 
able as  the  vena  cava  inferior.  This  important  venous  trunk  is  a  combined  vessel, 
comprising,  first,  a  part  of  the  hepatic  vein;  second,  a  large  channel  developed 
from  the  sinusoids  of  the  liver;  third,  the  upper  part  of  the  right  subcardinal  vein, 
and,  fourth,  the  lower  part  of  the  right  cardinal. 

The  Lymphatic  System. 

The  lymph  vessels  arise  in  connection  with  the  veins  and  are  probably  out- 
growths of  the  vascular  endothelium,  although  some  authorities  state  that  they  may 
begin  as  sub-endothelial  spaces.  They  may  be  recognized  by  their  very  delicate 
but  distinct  endothelial  walls,  thus  differing  from  the  mesenchymal  spaces-,  and  by 
the  complete  or  almost  complete  absence  of  blood-corpuscles  within  them.  They 
probably  end  blindly.  They  appear  relatively  late,  for  they  do  not  develop  until 
after  the  limb  buds  are  well  advanced  (pigs  of  14  mm.).  The  first  lymph-vessels 
develop  from  the  anterior  cardinals  in  the  cervical  region,  and  rapidly  fuse  to 
make  a  pair  of  very  large  vesicles,  the  jugular  lymph-sacs  (Fig.  60,  S.l.j],  which 
are  closely  applied  to  the  veins.  Each  jugular  lymph-sac  empties  into  the  cardinal 
vein  near  its  junction  with  the  subclavian  through  a  valve-like  orifice.  Whether 
this  connection  is  primary  or  secondary  is  still  uncertain.  From  each  sac,  narrow 
vessels  bud  out  into  the  mesenchyma,  anastomose  with  one  another,  and,  by  spread- 
ing more  and  more,  produce  the  lymphatic  system  of  the  neck  and  head.  Sub- 
sequently, the  jugular  sac  is  resolved  into  the  deep  cervical  plexus'  of  lymphatics. 
Similar  lymph-sprouts  in  slightly  older  embryos  produce  a.  more  irregular  median 
mesenteric  sac  (Fig.  60,  S.l.m)  just  below  the  renal  anastomosis,  R.A;  and  also 
another  sac,  Cis,  dorsal  to  the  first.  The  mesenteric  sac  sends  branches  into  the 
mesentery  to  drain  the  intestine,  and  at  the  same  time  joins  the  dorsal  sac  which 
enlarges,  becoming  the  cisterna  chyli.  Still  later  there  are  formed  in  a  similar 
manner  two  smaller  sacs  of  dense  plexuses,  termed  the  sciatic,  for  they  develop  in 
connection  with  the  root  of  the  sciatic  vein  (Fig.  60,  Sci).  The  sciatic  sacs  are 
produced  later  than  the  other  four.  Like  the  jugular,  the  four  later  formed  sacs 
serve  as  centers  of  growth  for  the  lymph-vessels.  In  addition,  other  lymphatics 
develop  from  other  veins.  Especially  notable  are  the  sprouts  from  the  azygos 
vein,  which  unite  and  ultimately  give  origin  to  the  main  lymphatic  trunk,  the 
ductus  thoracicus,  which  joins  tailward  the  cisterna,  headward  the  left  jugular  sac. 

The  lymph-glands  make  their  first  appearance  considerably  later  than  the 
vessels  (rabbits,  25-30  mm.;  human  embryos,  30-45  mm.).  Each  appears  be- 
tween a  vein  and  a  lymph-vessel  and  is  recognizable  as  mesenchyma  crowded 
with  young  leucocytes.  The  glands  are  very  small  at  first,  but  are  quite  sharply 
circumscribed. 


106 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


FIG.  60. — PIG  OF  20  MM.  SERIES  59.  RECONSTRUCTION  OF  THE  LYMPH-VESSELS  AND  PART  OF  THE  VENOUS 

SYSTEM.  BY  F.  T.  LEWIS. 

Az,  Azygos  vein  (remnant  of  the  posterior  cardinal).  Br,  Brachial  vein.  C.C,  Common  cardinal  vein.  Ce, 
Cephalic  vein.  Cis,  Cisterna  chyli.  Ex.J,  External  jugular  vein.  Fe,  Femoral  vein.  G,  Gastric  vein. 
In.J,  Internal  jugular  vein.  R.A,  Renal  anastomosis  between  the  subcardinal  veins.  Sci,  Sciatic  vein.  S.l.j, 
Jugular  lymph-sac.  S.l.m,  Mesenteric  lymph-sac.  S.M,  Superior  mesenteric  vein.  Th.ep,  Thoraco-epigastric 
vein.  V,  Omphalo-mesaraic  or  vitelline  vein.  V.C.I,  Vena  cava  inferior.  X  8  diams. 


THE  PANCREAS. 


107 


The  Liver. 

When  the  omphalo-mesaraic  veins,  the  first  large  veins  to  appear,  are  developed, 
they  are  situated  in  the  splanchnopleure  and  join  the  heart.  They  are  of  such 
large  size  as  to  cause  a  projection  into  the  coelom.  This  projection  is  the  septum 
transversum  (p.  87).  As  shown  in  the  diagram  (Fig.  16),  the  entoderm  of  the 
digestive  canal  of  the  head  of  the  embryo  passes  over  behind  the  pericardial  cavity 
and  behind  the  septum  transversum  into  the  yolk-sac.  Out  of  the  entoderm  cover- 
ing the  septum  transversum  on  its  caudal  side,  the  anlage  of  the  liver  is  developed 
(Fig.  25,  fo).  This  anlage  is  produced  by  a  rapid  proliferation  of  the  entodermal 
cells,  and  they  grow  toward  the  space  occupied  by  the  omphalo-mesaraic  veins 
(Fig.  157).  An  intergrowth  of  the  liver  cells  and  of  the  endothelium  of  the  veins 
takes  place.  The  cavity  of  the  veins  becomes  subdivided  into  smaller  blood  chan- 
nels, which  we  call  sinusoids  to  distinguish  them  from  capillary  vessels.  The  liver 
cells  arrange  themselves  in  the  form  of  cords  which  are  termed  the  hepatic  cylinders. 
Each  hepatic  cylinder  is  closely  invested  by  the  venous  endothelium.  The  liver 
consists  at  first  only  of  hepatic  and  endothelial  cells  and  is  situated  in  the  septum 
transversum. 

When  the  liver  becomes  larger,  it  protrudes 
from  the  septum  transversum,  but  does  not 
separate  from  it,  so  that  in  the  adult  the  liver  is 
always  found  attached  to  the  diaphragm,  which 
is  merely  the  modified  septum  transversum. 


The  Pancreas. 

The  pancreas  is  a  double  organ,  for  it  arises 
from  two  distinct  anlages:  first,  the  dorsal  pan- 
creas, which  appears  as  an  entodermal  evagination 
on  the  dorsal  side  of  the  duodenum  (Fig.  61, 
Panc.d),  soon  branches,  and,  continuing  to  grow, 
rapidly  develops  into  the  body  and  tail  of  the 
adult  organ.  Its  connection  with  the  intestine 
becomes  the  dorsal  pancreatic  duct  (ductus  San-  M, ',  4  Cords  of  hepatic  cells.  Panc.d, 

- 

torini).      The    ventral    pancreas    appears    a    little 

later   and    grows   more    slowly   than   the   dorsal.        It 
arises     as     an     Outgrowth     (Fig.     6l,    Panc.v}     from 

the  entodermal  ductus  choledochus.  It  develops 
into  the  head  of  the  pancreas  and  dorsal  pancreatic  duct  (ductus  Wirsungi).  The 
two  anlages  expand,  meet  (Fig.  62),  and  unite.  In  the  pig,  the  dorsal  anlage  is 
farther  from  the  stomach  than  the  ventral,  but  in  man  it  is  nearer  the  stomach. 

In  the  pig,  the  ventral  pancreatic  duct  is  obliterated  and  the  dorsal  duct  alone 
is  normally  persistent  in  the  adult.  In  man,  on  the  contrary,  the  dorsal  duct  is 
normally  obliterated  and  the  ventral  duct  persists.  Occasionally  both  ducts  are 


Ves-fel* 


FIG.  61. — PIG  EMBRYO  OF  5.5  MM.  SERIES 
915.  WAX  RECONSTRUCTION  OF  THE 
STOMACH  AND  PANCREATIC  ANLAGES 
BY  F.  W.  THYNG. 


Dorsal  pancreas.  Panc.v,  Ventral  pan- 
creas. St,  Stomach.  Ves.fel,  Gall- 
bladder, x,  Ventral  process  of  the 
dorsal  pancreas  (situated  on  the  right  of 
the  portal  vein).  X  55  diams. 


108 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


found   in   human   adults,    and   in   such   cases   the   dorsal   duct   has   been   commonly 
termed   "accessory." 

The  cords  of  pancreatic  cells  are  at  first  solid,  but  they  for  the  most  part 
acquire  lumina,  thus  becoming  epithelial  gland-tubes.  The  areas  of  Langerhans 
are  small  patches  of  pancreatic  cell  cords  found  in  the  adult  without  any  lumina. 


FIG.  62. — PIG  EMBRYO  OF  20  MM.  SERIES  60.  WAX  RECONSTRUCTION  or  THE  DUODENUM  AND  PANCREATIC 

ANLAGES  BY  F.  W.  THYNG. 

Div,  Duodenal  diverticulum.  D.chol,  Ductus  choledochus.  D.panc.d,  Ductus  pancreatis  dorsalis.  D.pqnc.v, 
Ductus  pancreatis  ventralis.  Pane. ace,  Pancreas  accessor! um  (anomaly).  Panc.d,  Pancreas  dorsale.  Panc.v, 
Pancreas  ventrale.  St,  Stomach,  x,  Ventral  process  of  the  dorsal  pancreas,  on  the  right  of  the  portal  vein. 
X  55  diams. 

The  Excretory  Organs. 

No  less  than  three  distinct  excretory  organs  are  known  to  occur  in  vertebrates. 

Of  these,  the  first  is  termed  the  pronephros,  or  head  kidney,  on  account  of  its 
position  toward  the  head  and  in  the  neighborhood  of  the  heart.  It  is  well  developed 
and  the  only  excretory  organ  in  certain  fishes  and  in  the  early  larval  stages  of 
amphibia.  In  elasmobranchs,  which  occupy  in  this  respect  an  exceptional  position, 
and  in  amniota  it  exists  in  a  rudimentary  form  only,  except  as  to  its  duct,  which 
plays  an  important  role  in  the  further  development.  The  pronephros  consists  of 


THE  EXCRETORY  ORGANS. 


109 


few  epithelial  tubes  which  take  a  somewhat  twisting  course,  but  may  be  said  to 
run,  in  general  terms,  transversely.  Each  tube  begins  with  a  ciliated  funnel-shaped 
opening  (Fig.  63,  /)  not  far  from  the  median  line  of  the  embryo,  and  ends,  after 
a  more  or  less  contorted  course,  in  a  longitudinal  duct,  which,  after  receiving  all 
of  the  tubules,  runs  toward  the  posterior  end  of  the  embryo  and  opens  into  the 
extremity  of  the  entodermal  or  digestive  canal.  Opposite  the  funnels,  and  separate 
from  the  pronephros  proper,  there  is  a  so-called  glomus  (Fig.  63,  gl),  which  is  a 
projection  of  not  inconsiderable  size  from  the  mesentery.  When  fully  developed 
the  glomus  contains  a  rich  network  of  blood-capillaries,  so  that  it  somewhat  resem- 
bles the  glomerulus  of  the  kidney.  The  circulation  of  the  pronephros  is  sinusoidal. 
The  second  of  the  excretory  organs  is  termed  the  mesonephros,  Wolffian  body, 
or  fetal  kidney.  It  is  the  only  excretory  organ  in  elasmobranchs.  In  adult  am- 


•nch 


•Ec 


FIG.  63. — FROG  (RANA  TEMPORARIA)  TADPOLE  OF  12  .o  MM.     CROSS-SECTION  OF  THE  PRONEPHRIC  REGION. 

nch,  Notochord.     m,  Muscles.    /,  Pronephric  funnel,     v,  Blood-vessel.     EC,  Ectoderm,     t,  Pronephric  tubule. 

gl,  Glomus.    Lu,  Lung.     X  90  diams. — (After  M.  Furbringer.) 


phibians  it  replaces  the  pronephros,  which  i§  purely  a  larval  structure.  It  is  pres- 
ent in  the  embryos  of  all  amniota,  but  undergoes  a  partial  degeneration  before 
adult  life,  being  itself  replaced  in  adult  amniota  by  the  true  kidney.  The  meso- 
nephros resembles  somewhat  the  pronephros,  especially  as  found  in  the  ichthyopsida. 
It  occupies  a  much  larger  region  of  the  body  than  the  pronephros.  It  has  no 
glomus  associated  With  it,  but  each  tubule  contains  a  glomerulus  very  similar  in 
its  general  structure  to  the  glomerulus  of  a  true  kidney.  In  the  ichthyopsida  each 
tubule  begins  with  a  ciliated  funnel,  and,  after  making  several  coils,  opens  into  the 
pronephric  duct.  In  the  amniota  the  mesonephros,  or,  as  it  is  more  commonly 
called  in  these  animals,  the  Wolffian  body,  is  essentially  an  embryonic  structure. 
Its  tubules,  however,  do  not  have  at  any  stage  the  ciliated  funnels  to  be  found  in 
amphibia  and  fishes,  but  they  have  glomeruli  and  they  open  into  the  pronephric 


110  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

duct,  which,  on  account  of  its  relations  to  the  organs  is  in  this  type  more  com- 
monly spoken  of  as  the  Wolffian  duct.  The  circulation  of  the  organ  is  sinusoidal. 
Further  details  are  given  in  the  practical  part  in  connection  with  study  of  the  pig 
embryos,  pages  252  and  306. 

The  third  of  the  excretory  organs  is  termed  the  metamphros,  or  true  kidney. 
In  the  mammalian  embryo,  after  the  Wolffian  body  has  acquired  a  considerable 
development,  there,  appears  a  small  outgrowth  of  the  Wolffian  duct  at  a  point  near 
the  junction  of  the  duct  with  the  allantois.  It  extends  dorsad  and  cephalad  (Fig. 
172,  ki),  and  may  be  termed  the  renal  evagination.  Its  blind  end  expands  to  be- 
come the  pelvis  of  the  kidney,  while  its  stalk  remains  narrower  and  is  converted 
into  the  ureter,  by  which  the  urine  is  conveyed  from  the  pelvis  to  the  bladder 
(allantois).  By  outgrowths  of  the  epithelium  of  the  pelvis  the  collecting  tubules  are 
developed.  Around  the  pelvis  appears  an  envelope  of  special  cells,  easily  recognized 
by  their  darker  staining  (Fig.  210).  These  cells  are  thought  to  be  derived  from  the 
nephrotomes  of  the  segments  of  the  renal  region,  which  have  lost  their  epithelial 
arrangement  and  have  migrated  to  form  the  envelope.  The  cells  gather  themselves 
gradually  into  small  clumps,  .the  number  of  clumps  continuing  to  increase  during  a 
long  period.  Each  clump  assumes  an  epithelial  arrangement  and  acquires  a  lumen 
and  elongates  into  a  tubular  form.  The  tubule  elongates,  one  end  joins  a  collect- 
ing tubule,  and  the  lumina  of  the  two  structures  become  continuous.  The  other 
end  of  the  tubule  remains  closed  and  is  converted  into  the  renal  corpuscle.  The 
tubule  now  grows  rapidly  in  length  and  becomes  very  irregular  in  its  course;  the 
part  which  joins  the  collecting  tubule  becomes  the  proximal  convoluted  tubule;  the 
part  which  joins  the  renal  corpuscle  becomes  the  distal  convoluted  tubule,  and  the 
middle  part  between  these  two  becomes  the  loop  of  Henle. 

The  Urogenital  Ducts. 

The  genital  and  excretory  organs  always  develop  together  and  constitute  the 
urogenital  system.  The  genital  glands  are  always  distinct,  but  the  primary  excre- 
tory or  Wolffian  duct,  after  producing  the  outgrowth  to  form  the  renal  anlage  (p. 
309),  is  transformed  into  the  permanent  male  duct.  The  other  primary  canal  is' 
termed  the  Mullerian  duct.  It  is  exclusively  genital,  for  with  its  mate  it  develops 
the  uterine  tubes,  the  uterus,  and  the  vagina,  but  in  the  male  it  becomes  vestigial. 

The  Wolffian  duct  is  the  excretory  canal  of  the  mesonephros  (p.  109).  It 
extends  along  the  ventral  surface  of  the  organ;  the  mesonephric  or  Wolffian  tubules 
acquire  openings  into  it,  and  it  itself  opens  at  its  caudal  end  into  the  base  of  the 
allantois.  It  serves  for  a  time  as  a  true  excretory  duct,  but  loses  this  function 
when  the  mesonephros  degenerates.  It  meanwhile  acquires  a  secondary  connection 
with  the  testis  by  means  of  some  of  the  Wolffian  tubules  in  the  neigrfborhood  of  the 
genital  gland.  The  tubules  in  question  acquire  a  connection  with  the  sexual 
cords  of  the  testis  and,  when  the  cords  become  seminiferous  tubulef,  the  Wolffian 
tubules  are  ready  to  conduct  the  semen  to  the  Wolffian  duct  (vas  defer  ens).  The 


THE  ALL  AN  TO  IS.  Ill 

course  of  the  Wolffian  duct  is  changed  during  fetal  life  by  the  migration  of  the 
testis  from  its  original  abdominal  position  into  the  scrotum.  In  the  female  the 
Wolffian  duct  becomes  vestigial. 

The  Mullerian  ducts  develop  much  later.  They  may  be  found  in  the  12  mm. 
pig  as  two  short  funnels  formed  by  the  mesothelium  and  situated  on  the  ventral 
side  of  the  mesonephros  near  the  septum  transversum  (diaphragm).  The  funnels 
point  backward  and  grow  into  tubes,  which  run  on  the  ventral  side  of  the  Wolffian 
duct  and  presently  connect  with  and  open  into  the  base  of  the  allantois.  The 
pelvic  portions  of  the  two  Mullerian  ducts  approach  one  another  in  the  median 
line  and  in  the  female  they  fuse,  making  a  median  epithelial  canal,  the  anlage  of 
the  uterus  and  vagina.  The  original  entrance  to  the  canal  persists  as  the  fimbriate 
opening  and  the  stretch  of  the  original  canal  between  the  funnel  and  the  uterus 
becomes  the  uterine  tube. 

The  Allantois. 

The  allantois  is  a  diverticulum  of  the  entodermal  canal,  and  is,  therefore,  lined 
by  entodermal  epithelium  (Fig. -21).  It  arises  on  the  ventral  side  of  the  caudal  end 
of  the  embryo  in  proximity  to  the  anal  plate.  In  its  development  we  can  distin- 
guish two  main  types.  The  first  type  is  illustrated  by  the  sauropsida  and  the  un- 
gulates. In  them  it  grows  out  and  rapidly  enlarges  so  as  to  form  a  vesicle  of 
considerable  size  and  connected  with  the  embryo  by  means  of  a  narrow  hollow 
stalk.  When  the  allantois  develops  according  to  this  type,  it  is  spoken  of  as  free, 
because  it  has  no  connection  with  the  extra-embryonic  somatopleure  (chorion  and 
amnion).  This  form  of  the  allantois  may  be  readily  observed  in  chick  embryos, 
for  by  the  fourth  day  it  has  become  a  considerable  rounded  vesicle  which  lies 
in  the  extra-embryonic  ccelom  between  the  yolk-sac  and  the  extra-embryonic  soma- 
topleure or  membrana  serosa.  During  the  fifth  day  it  rapidly  enlarges,  and  at  the 
beginning  of  the  sixth  day  is  nearly  or  quite  as  large  as  the  head  of  the  embryo. 
In  ungulates  the  growth  of  the  free  allantois  begins  very  early  and  becomes  enor- 
mous. Its  principal  expansion  is  sideways,  that  is  to  say,  at  right  angles  to  the 
axis  of  the  embryo,  and  it  becomes  a  large  sac,  very  much  larger,  indeed,  than  the 
entire  embryo. 

The  second  type  of  allantois  occurs  in  the  placental  mammals  of  unguiculate 
series  and  is  not  known  to  occur  in  'any  species  of  the  ungulate  type.  In  probably 
all  unguiculates  the  posterior  end  of  the  body  has  a  prolongation  which  is  known 
as  the  body-stalk  (Fig.  69,  b.s).  Into  this  body-stalk  the  diverticulum  consti- 
tuting the  allantois  extends  (Fig.  80,  All,  and  Fig.  25,  All).  The  entoderm  of 
the  allantois  is  surrounded  by  mesoderm,  which  is  present  in  the  body-stalk  in  con- 
siderable volume.  On  the  outer  surface  there  extends  a  layer  of  ectoderm,  so  that 
the  three  germ-layers  enter  into  the  formation  of  the  body-stalk  as  they  do  into  the 
formation  of  the  embryo.  These  relations  are  illustrated  by  the  diagram  (Fig.  64). 
By  means  of  the  body-stalk  a  connection  is  established  between  the  embryo  and 


112 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


Cce. 


Cho. 


Yk. 


the  extra-embryonic  somatopleure  or  primitive  chorion,  Cho.  Later,  when  the  for- 
mation of  the  amnion  is  completed,  the  essential  relations  are  found  to  be  as 
illustrated  by  the  diagram  (Fig.  64,  B).  The  amnion  arises  from  the  distal  end 
of  the  body-stalk,  but  the  body-stalk  retains  its  connection  with  the  chorion.  When 
the  allantois  becomes  free,  the  connection  with  the  chorion  is  entirely  lost.  The 
maintenance  of  that  primitive  connection  in  the  unguiculates  is  to  be  regarded  as  a 
new  modification  of  the  relations  of  the  embryonic  appendages,  evolved  only  in 

the  higher  animals.  The  maintenance  of 
that  connection  makes  possible  the  modi- 
fication in  the  structure  of  the  chorion, 
which  is  of  the  greatest  morphological 
importance.  This  modification  is  the 
development  of  the  blood-vessels  in  the 
chorion.  The  anlages  of  these  blood- 
vessels are  outgrowths  of  the  embryonic 
angioblast.  They  appear  so  as  to  form 
four  vessels  which  grow  through  the 
length  of  the  body-stalk  in  the  neighbor- 
hood of  the  allantoic  diverticulum.  Two 
Am.  of  these  vessels  are  veins  and  two  are 
Emb  arteries.  They  are  termed  the  umbilical 
vessels.  The  umbilical  veins  at  the 
Cce.  embryonic  end  of  the  body-stalk  enter 
the  somatopleure  of  the  embryo  (Fig. 
Yk.  186,  V.  U.S.  ,  V.  U.D),  through  which 
they  make  their  way  toward  the  heart 
(Fig.  93,  Alv).  The  umbilical  arteries 
enter  the  body  of  the  embryo,  pass 
caudad  alongside  the  allantois  (Fig.  210, 

A,  Before,  B,  after  the  formation  of  the  amnion.     All,  A.um),    Curve  past     the     cloaca     onto     the 

Entodermal  allantois.     Am,  Amnion.     b.s,  Body-  dorsal   side   of  the   body    (Fig.    169,   A  Mm), 
stalk.      Cho,      Chorion.      Cos,     Extra-embryonic 

coelom.     Emb,    Anterior    end    of    embryo.     Yk,  and    Jom    the  Caudal     Cnd    °f    the    a°Fta> 

Yolk-sac.  so    that   they   may  be  termed  the  termi- 

nal   branches    of    the    embryonic     aorta. 

In  early  stages  they  are  the  largest  branches  which  the  aorta  has.  At  the  distal 
end  of  the  body-stalk  the  four  vessels  enter  the  mesoderm  of  the  chorion,  there 
branch  abundantly,  and  produce  a  rich  network  of  blood-vessels  throughout  the 
entire  membrane.  The  unguiculate  mammals,  therefore,  are  characterized  by  this 
special  feature,  the  possession  of  the  body-stalk  which  contains  the  allantoic  diver- 
ticulum and  gives  access  for  the  blood-vessels,  and  therefore  also,  of  course,  for  the 
blood,  to  the  chorion,  which  thus  becomes  vascular.  In  all  other  amniota  the 
chorion  is  without  blood-vessels. 


Cho. 


FIG.  64. — DIAGRAMS  ILLUSTRATING  THE  RELATIONS  OF 
THE  ALLANTOIS  IN  UNGUICULATE  MAMMALS. 


THE  ALL  AN  TO  IS.  113 

The  size  of  the  allantoic  cavity  in  unguiculates  varies  considerably.  In  man 
it  is  minimal,  forming  only  a  long  and  very  narrow  tube  (compare  Fig.  66,  All). 
In  rodents  it  expands  somewhat,  but  it  never  becomes  free  in  the  sense  that  it  is 
separated  from  the  body-stalk,  although  it  may  acquire  a  partial  independence.  In 
this  case  it  may  also  become  more  or  less  vascular  by  the  development  of  branches 
from  the  umbilical  arteries  and  veins  around  the  allantois. 

In  those  animals  in  which  the  allantois  is  free,  the  umbilical  arteries  and  veins 
have  all  their  branches  in  the  allantois,  there  being  no  body-stalk.  The  embryo 
is  without  connection  with  the  chorion,  and,  therefore,  these  vessels  in  their  rami- 
fications are  restricted  to  the  allantois. 

Relations  of  the  Allantois  to  the  Chorion  in  Ungulates. — Since  the  true  chorion  is 
the  outermost  of  the  fetal  envelopes,  it  alone  can  come  in  contact  with  the  walls 
of  the  uterus.  All  placental  developments,  therefore,  necessarily  depend  upon  the 
chorion.  Now,  in  ungulates,  where  the  chorion  is  without  blood-vessels,  there  is 
no  circulatory  apparatus  to  transfer  any  nutritive  material,  which  may  be  taken  up 
by  the  chorion  from  the  uterus  to  the  embryo,  until  a  second  union  takes  place 
between  the  vascularized  allantois  and  the  chorion.  The  inner  surface  of  the 
chorion  and  the  outer  surface  of  the  allantois  are  both  mesodermic.  The  two 
mesodermic  layers  come  into  contact  with  one  another  and  unite  loosely.  The 
vessels  of  the  allantoic  mesoderm  are  thus  brought  into  physiological  union  with 
the  chorion,  but,  being  allantoic  vessels,  they  are,  of  course,  morphologically  differ- 
ent from  the  chorionic  vessels  of  unguiculate  mammals.  These  considerations 
demonstrate  that  the  ungulate  placenta  is  allantoic  rather  than  chorionic,  and  is, 
morphologically  speaking,  essentially  different  from  the  true  chorionic  placenta, 
which  can  be  developed  only  in  those  animals  and  embryos  which  have  a  perma- 
nent body-stalk. 

The  simple  relations  of  the  chorion  in  the  Ungulata  to  the  uterine  wall  is 
illustrated  by  the  accompanying  figure  65,  which  shows  a  portion  of  the  chorion 
of  a  pig  embryo  of  15  mm.,  together  with  the  surface  of  the  uterus  to  which  it  was 
fitted.  The  two  membranes  were  accidentally  separated  in  the  preparation.  The 
chorion  consists  of  a  layer  of  cylinder  epithelial  cells,  EC,  each  of  which  can  be 
distinctly  made  out,  and  of  a  layer  of  mesoderm,  Mes,  containing  only  few  cells 
and  blood-vessels,  two  of  which,  Ve,  are  shown  in  the  section;  the  mesodermic 
cells  are  a  little  more  crowded  near  the  epithelium.  Each  ectodermal  cell  is 
distinctly  marked  off  from  its  neighbors  by  a  line.  The  protoplasm  stains  some- 
what; the  nuclei  are  slightly  oval  and  granular,  and  are  situated  near  the  middle 
of  the  cells.  The  top  of  each  cell  is  concave.  The  uterine  epithelium,  Ut.Ep, 
resembles  in  the  general  form  of  its  cells  and  in  the  character  of  its  protoplasm 
the  chorionic  ectoderm,  but  differs  from  it  in  that  each  cell  has  a  convex  free  end, 
and,  further,  in  that  the  nuclei  of  the  cells  are  situated  near  the  top  of  the  layer. 
When  the  relations  of  the  two  epithelia  have  not  been  disturbed,  it  is  readily 
observed  that  the  concavity  of  each  chorionic  ectodermal  cell  receives  the  convex 


114 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


end  of  the  uterine  epithelial  cell,  so  that  the  two  layers  are  closely  fitted  together, 
cell  for  cell. 

The  Bladder.— The  allantois  extends  from  the  cloaca  to  the  umbilicus,  and 
beyond  the  umbilicus  into  the  umbilical  cord.  It  comprises,  therefore,  an  embry- 
onic and  an  extra-embryonic  portion.  The  former  is  the  anlage  of  the  urogenital 
sinus,  the  urethra,  and  the  bladder.  The  embryonic  portion  is  always  united  to 
the  abdominal  wall  (Fig.  210),  the  mesenchyma,  which  surrounds  the  entodermal 
allantois,  All,  and  the  umbilical  arteries,  S.um,  being  fused  in  the  mid-ventral  line 


Ut.Ep. 


Conn. 


EC. 


Mes. 


FIG.  65. — PIG,  15.0  MM.,  SERIES  135,  SECTION  58,  TO  SHOW  THE  RELATIONS  OF  THE  CHORION  TO  THE  UTERUS. 
Conn,  Connective  tissue  of  the  uterus.     EC,  Chorionic  ectoderm.     Mes,  Choriomc  mesoderm.     Ut.Ep,  Uterine 
epithelium.     Ve,  Chorionic  blood-vessel.      X  350  diams. 

with  the  mesenchyma  of  the  somatopleure.  This  connection  is  retained  throughout 
life.  The  opening  of  the  Wolffian  ducts  into  the  allantois  is  established  very 
early.  The  ureters  at  first  open  into  the  Wolffian  ducts,  but  they  soon  migrate 
so  as  to  open  separately  directly  into  the  allantois  (bladder)  above  the  ducts. 

The  Trophoderm. 

Trophoderm  is  the  name  applied  to  the  special  layer  of  cells  developed  on  the 
outer  surface  of  the  ectoderm  of  the  mammalian  blastodermic  vesicle.  It  has  as 
yet  been  observed  only  in  unguiculates.  The  trophoderm  layer  may  be  devel- 
oped over  the  entire  surface  of  the  ovum,  as  in  man,  or  over  only  a  portion 
thereof,  as  in  the  rabbit  and  cat.  Its  principal  known  function  is  to  destroy  the 
tissues  of  the  uterus  of  the  mother  with  which  it  comes  in  contact.  The  destruc- 
tion of  the  tissue  is  supposed  to  serve  two  purposes:  First,  to  supply  nutrition  to 
the  embryo.  It  is  from  this  supposed  function  that  the  layer  derives  its  name  of 


THE  UMBILICAL  CORD.  .  115 

trophoderm.  Second,  to  secure  the  attachment  of  the  ovum  to  the  wall  of  the 
uterus.  This  preliminary  attachment  is  called  the  implantation  of  the  ovum.  In 
some  cases  the  trophoderm  is  developed  very  early  over  the  surface  of  the  ovum 
(Fig.  74),  appearing  almost  as  soon  as  the  stage  of  the  blastodermic  vesicle  is 
reached,  and  while  the  vesicle  is  very  small.  In  such  cases  the  ovum  creates  a 
space  for  itself  by  dissolving  away  the  epithelium  and  connective  tissue  at  a  small 
spot  on  the  uterine  surface,  making  a  cavity  in  which  the  ovum  lodges.  In  other 
cases  the  trophoderm  is  developed  later  and  does  not  appear  over  the  whole  of  the 
blastodermic  vesicle.  The  area  over  which  it  exists  in  such  cases  is  called  the 
placental  area  (compare  pages  127  and  179).  The  trophoderm  in  these  forms 
unites  very  closely  indeed  with  the  surface  of  the  uterus  (Fig.  37,  Tro)  and  the 
uterine  tissues  undergo  degeneration  and  resorption.  We  may  regard  as  the  first 
step  toward  the  production  of  the  placenta  proper  the  disappearance  of  the  tropho- 
derm. Our  knowledge  of  its  disappearance  is  incomplete,  but  it  is  probable  that 
it  is  due  to  a  transformation  of  the  cells  of  the  trophoderm,  associated  with  con- 
temporaneous modifications  of  the  chorionic  membrane,  of  which  the  general  result 
may  be  said  to  be  formation  of  the  chorionic  villi  which  constitute  the  fetal  pro- 
tion  of  the  placenta.  The  modified  trophoderm  cells  are  supposed  to  enter  into 
the  formation  of  the  ectodermal  covering  of  these  villi. 


The  Umbilical  Cord. 

The  umbilical  cord  may  be  best  defined  as  the  tissues  connecting  the  body 
proper  of  the  embryo  with  the  amnion.  It  accordingly  includes  a  portion  of  the 
body-stalk  and  a  certain  extent  of  the  body-wall  or  somatopleure.  In  early  stages 
we  can  hardly  speak  of  an  umbilical  cord,  because  the  amnion  arises  close  to  the 
embryo  (Fig.  83).  As  development  progresses  the  body-stalk  lengthens  out  and  the 
amnion  arising  from  it  recedes  farther  and  farther  from  the  embryo,  this  recession 
being  assisted  by  a  growth  of  the  somatopleure  which  leads  to  the  formation  of  the 
umbilical  cord  proper.  By  this  means  a  tubular  structure  is  produced,  the  cavity 
of  the  tube  being  a  prolongation  of  the  coelom  of  the  embryo.  During  the  first 
develbpment  of  the  umbilical  cord  the  neck  of  the  yolk-sac  becomes  constricted  and 
very  much  lengthened  out,  forming  the  yolk  or  vitelline  stalk.  The  yolk-stalk 
springs  within  the  embryo  from  the  wall  of  the  intestine,  runs  through  the  coelom 
of  the  umbilical  cord,  and  makes  its  exit  beyond  the  amnion,  as  shown  in  figure 
102.  The  yolk-sac  proper  still  occupies  its  original  position  between  the  amnion 
and  chorion.  The  student  should  note  carefully  that  the  umbilical  cord  is  never 
covered  by  the  amnion,  for  it  has  unfortunately  been  often  stated  that  it  is  so 
covered.  Ah  idea  of  the  relations  can  be  gathered  from  cross-sections  of  the  cord 
(Fig.  66).  The  ccelom,  Cw,  is  a  large  cavity  and  contains  the  yolk-stalk,  Y,  with 
two  blood-vessels,  but  with  its  entodermal  cavity  entirely  obliterated.  Above  the 
body-cavity  is  the  duct  of  the  allantois,  All,  lined  by  entodermal  epithelium,  and  in 


116 


.THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


its  neighborhod  are  two  arteries  and  a  single  vein.  In  yet  earlier  stages  there  are 
two  veins.  The  outer  surface  of  the  section  is  bounded  by  ectoderm.  The  further 
development  of  the  cord  depends  upon  the  growth  of  the  connective  tissue  and 
blood-vessels,  the  abortion  first  of  the  coelom,  later  of  the  yolk-stalk,  and  lastly  of 
the  allantoic  duct.  Remnants  of  the  allantoic  epithelium  are,  however,  often  found 
in  the  umbilical  cord  even  at  birth.  There  occurs  also  a  further  differentiation  of 
the  connective  tissue  and  of  the  entoderm. 

The  umbilical  cord  is  characteristic  of  mammals.  It  varies  greatly  in  length. 
In  the  pig  it  is  very  short.  In  man  it  attains  great  length  and  size,  becoming  at 
full  term  about  55  cm.  in  length  and  12  mm.  in  thickness.  When  fully^  developed 
the  human  cord  has  a  whitish  color  and  presents  a  twisted  appearance  somewhat 


FIG.  66. — SECTIONS  OF  Two  HUMAN  UMBILICAL  CORDS. 

A,  From  an  embryo  of  21  mm.;  B,  from  an  embryo  of  sixty-four  to  sixty-nine  days.     All,  Allantois.     Ar,  Umbilical 
artery.     Cce,  Coelom.     v,  Umbilical  vein.     Y,  Yolk-stalk. 


like  a  rope.  Its  surface  is  smooth  and  glistening.  The  attachment  of  the  cord  to 
the  embryo  is  known  as  the  umbilicus.  Its  attachment  to  the  chorion  is  in  the  pla- 
cental  region  (chorion  frondosum). 

The  twisting  of  the  cord  is  well  marked  externally  at  the  time  of  birth  by  the 
spiral  ridges,  within  each  of  which  a  large  blood-vessel  runs.  The  number  of  spirals 
varies  from  3  to  32,  the  turns  beginning  at  the  embryo,  and,  though  usually  from 
left  to  right,  are  sometimes  from  right  to  left.  The  twisting  begins  about  the  middle 
of  the  second  month.  Its  cause  is  unknown,  but  there  is  no  reason  to  assume  that 
it  is  due  to  revolutions  of  the  embryo.  The  cord  is  covered  by  a  layer  of  epithe- 
lium which  is  continuous  at  the  distal  end  with  the  epithelium  of  the  amnion,  and 
at  the  proximal  end  with  the  epidermis  of  the  embryo.  The  cord  contains  typically 
no  capillaries,  and,  except  in  the  immediate  neighborhood  of  the  embryo,  no  nerve- 
fibers. 


THE  CH ORION  AND  AM N ION.  117 

The  Chorion  and  Amnion. 

These  are  two  membranes  which  always  surround  the  embryo,  the  chorion 
being  the  outer,  the  amnion  the  inner,  membrane  of  the  two.  Morphologically,  they 
are  modifications  of  the  extra-embryonic  somatopleure.  The  accompanying  diagrams 
render  this  clear.  In  figure  29,  we  see  that  the  cavity  of  the  mesoderm,  coe,  has  ex- 
tended completely  around  the  yolk.  There  is  a  layer  of  mesoderm,  represented  by 
a  dotted  line,  on  the  outside  of  this  cavity,  which  joins  with  the  overlying  ecto- 
derm, represented  in  the  diagram  by  a  continuous  line,  to  constitute  the  somato- 
pleure, Som.  In  figure  45,  we  see  the  somatopleure  folded  up  on  the  dorsal  side  of 
the  embryo;  the  leaf  of  the  fold  nearest  the  embryo  is  the  anlage  of  the  amnion, 
aw;  the  rest  of  the  extra-embryonic  somatopleure  is  the  anlage  of  the  chorion'  In 
the  second  figure  of  the  diagram,  the  two  folds  have  met  above  the  embryo  and 
united,  thus  making  a  closed  inner  amniotic  and  a  closed  outer  chorionic  envelope. 
The  actual  appearances  of  two  such  stages  as  in  the  diagram  are  illustrated  by 
figures  38  and  47.  By  this  account  we  learn  that  the  two  envelopes  are  produced  by 
a  folding  of  the  somatopleure. 

When  we  come  to  study  the  development  of  mammals  in  detail,  we  discover 
that  there  are  many  remarkable  variations  in  the  early  development  of  the  amnion 
of  which  no  general  explanation  is  yet  possible;  but  inasmuch  as  the  folding  pro- 
cess is  the  only  one  in  Sauropsida,  and  also  occurs  in  many  mammals  of  different 
classes,  it  is  generally  assumed  to  be  the  primitive  method. 

In  man  the  development  of  the  chorion  and  amnion  differs  extremely  from  the 
scheme  given  above.  It  is  described  as  accurately  as  our  present  knowledge  per- 
mits in  Chapter  III. 

However  developed,  the  fetal  envelopes  present  certain  constant  characteristics: 
both  consist  of  ectoderm  and  mesoderm,  but  in  the  case  of  the  amnion  the  ecto- 
derm is  turned  toward  the  embryo,  whereas  the  chorionic  ectoderm  faces  the  out- 
side. The  cavity  between  the  amnion  and  the  embryo  becomes  filled  with  the 
amniotic  fluid,  which  serves  as  an  important  mechanical  protection  to  the  develop- 
ing embryo.  It  is  through  the  chorionic  ectoderm  only  that  the  ovum  can  come 
into  actual  contact  with  the  walls  of  the  uterus.  It  is  the  chorion  alone  which  is 
concerned  in  the  formation  of  the  true  placenta  (compare  Chapter  VII). 

The  amnion  is  a  thin,  pellucid,  non- vascular  membrane;  the  chorion  is  thicker, 
more  nearly  opaque,  and  has  in  man  and  all  nearly  related  animals  a  highly  de-. 
veloped  vascular  system. 


CHAPTER  III. 
THE  HUMAN  EMBRYO. 

Our  knowledge  of  the  early  stages  of  human  development  is  very  imperfect. 
Upon  the  fertilization  and  segmentation  of  the  ovum  in  man  there  are  no  obser- 
vations whatever  at  present.  It  is  not  even  known  exactly  how  long  the  ovum 
requires  for  its  passage  through  the  Fallopian  tube.  The  earliest  stages  of  which 
we  have  comparatively  adequate  accounts  are  those  represented  by  Peters's  ovum 
(1899)  and  Herzog's  (1909).  A  number  of  human  embryos  in  various  early  stages 
after  the  formation  of  the  medullary  canal  and  up  to  the  stage  with  four  aortic 
arches  have  now  been  reported  and  studied,  some  few  of  them  thoroughly  and 
carefully. 

Calculation  of  the  Age  of  the  Human  Embryo.  * 

The  age  of  the  embryo  must  be  reckoned  from  the  date  of  the  fertilization  of 
the  ovum,  which  presumably  occurs  in  man  in  the  upper  third  of  the  Fallopian 
tube.  It  may  be  that  ova  become  fertilized  at  various  epochs,  but  fail  to  continue 
their  development  except  when  the  fertilization  occurs  at  the  beginning  of  a  men- 
strual period.  Ovulation  occurs  at  all  periods,  but  most  frequently  about  the 
time  of  menstruation,  which  is  the  expression  of  structural  changes  in  the  uterus 
which  enable  the  ovum  to  implant  itself  in  the  uterine  wall.  Hence  only  when 
fertilization  coincides  with  the  beginning  of  menstruation  can  conception  follow 
with  the  result  that  the  menstrual  flow  is  stopped.  Accordingly,  the  age  of  the 
embryo  is  usually  to  be  reckoned  from  the  date  of  the  beginning  of  the  first 
menstrual  period  which  has  lapsed. 

Experience,  however,  shows  that  sometimes  conception  occurs  without  stopping 
the  menstrual  change  at  the  time,  but  eliminating  only  the  subsequent  periods, 
and  in  such  cases  the  age  must  be  estimated  from  the  beginning  of  the  last 
menstruation.  In  the  two  cases  the  age  of  the  embryo  would  differ  by  a  month 
(twenty-eight  days),  and  this  difference  is  so  great  that  it  obviates  errors  of 
estimate. 

Up  to  the  end  of  the  ninth  week  the  form  and  size  of  the  embryo  exhibit  a 
correlated  development,  so  that  generally  an  embryo  at  a  given  stage  of  develop- 
ment in  form  will  agree  with  its  fellows  in  size;  but  to  this  rule  there  are  not  in- 
frequently exceptions,  and  sometimes  an  embryo  is  found  much  larger  than  others 

118 


THE  CLASSIFICATION  OF  THE  EARLY  STAGES.  119 

at  the  same  stage.  Moreover,  the  variability  of  embryos  is  very  great,  for  in 
specimens  otherwise  alike  we  find  this  or  that  organ  advanced  or  retarded  in  its 
development  as  compared  with  the  embryo  as  a  whole.  Nevertheless  it  is  possible 
with  the  information  at  command  to  determine  with  tolerable  certainty  the  age  of 
an  embryo  within  two  days  plus  or  minus,  up  to  the  end  of  the  ninth  week.  For 
the  course  of  development  during  the  third  month  we  possess  as  yet  no  satisfactory 
data,  but  embryos  of  full  three  months  are  quite  frequently  obtained,  and  are  very 
characteristic  in  size  and  configuration  (see  page  156). 

F.    P.    Mall's    formula    for   calculating   the   age    of   human   embryos   is 

A/ioo  X  length  in  mm: 
The  length  is  measured  from  the  vertex  to  the  breech. 

The  Classification  of  the  Early  Stages. 

Any  attempt  to  divide  embryos  into  stages  must  necessarily  establish  artificial 
groups,  for  in  nature  there  is  no  demarcation.  Division  into  stages  is  for  con- 
venience, and  ought,  therefore,  to  be  made  by  natural  and  obvious  characteristics. 
It  seems  to  me  that  eleven  stages  may  be  conveniently  discriminated,  as  follows: 

First  Stage. — Segmentation  of  the  Ovum:  The  general  process  is  described  on 
pages  42  to  45.  There  are  no  observations  upon  this  stage  in  man,  or  any  primate, 
except  one  monkey's  ovum  in  the  four-cell  s  age  described  by  Selenka. 
*  Second  Stage. — Blastodermic  Vesicle:  The  general  development  of  the  blasto- 
dermic  vesicle  in  mammals  is  described  on  page  45.  Its  development  in  man  is 
unknown.  During  this  stage  the  embryonic  shield  is  differentiated.  An  ovum 
of  a  monkey  in  this  stage  is  described  on  page  127,  and  one  of  the  very  few 
known  human  ova  is  described  on  page  128. 

Third  Stage. — Primitive  Streak :  Two  human  ova  with  a  primitive  streak  be- 
fore the  formation  of  the  medullary  plate  have  been  observed.1  In  one  of  these, 
Frassi's  embryo,  the  diameter  of  the  entire  ovum  was  13  x  5  mm.  The  diameter 
of.  the  yolk-sac  1.9  x  0.9  mm.  The  embryonic  shield  was  1.17  mm.  long  by  0.6 
mm.  wide.  The  primitive  groove  is  shallow  and  occupies  about  half  the  length 
of  the  shield.  The  anterior  end  of  the  groove  marks  the  position  of  the  future 
neurenteric  canal;  its  posterior  end,  the  position  of  the  anal  plate. 

Fourth  Stage. — The  Medullary  Plate:  In  this  stage  there  are  several  embryos 
known.  In  all  of  them  the  amnion  and  chorion  are  already  differentiated.  There  is 
a  large  extra-embryonic  coslom.  The  chorionic  vesicle  is  rounded  and  somewhat 
flattened.  In  its  greatest  diameter  it  measures  from  8  to  10  mm.  It  is  beset 
with  short  branching  villi  which  are  present  over  the  entire  surface.  The  general 
relations  are  indicated  in  the  accompanying  diagram  (Fig.  69).  The  chorion  has  a 
distinct  epidermal  and  mesodermal  layer.  To  its  inner  surface  is  attached  the 
body-stalk  wihch  unites  the  embryo  and  chorion.  From  it  springs  the  amnion 

'The  Harvard  Embryological  Collection  has  an  embryo,  Series  825,  in  fine  preservation.    It  is  a  little  younger 
than  Frassi's.     It  is  hoped  to  publish  an  account  of  it  soon. 


120  THE  HUMAN  EMBRYO. 

covering  the  embryo,  which  measures  only  i.o  to  1.5  mm.,  and  from  the  ventral 
surface  of  the  embryo  arises  the  yolk-sac,  which  is  of  rounded  form  and  about 
equal  in  diameter  to  the  length  of  the  embryo. 

Fifth  Stage. — The  Medullary  Groove:  The  general  relations  of  the  embryo  and 
its  appendages  are  the  same  as  in  the  previous  stage  (compare  Figs.  82  and  25). 
In  the  cases  recorded  the  chorionic  vesicle  varied  greatly  in  size.  It  bore  villi 
over  its  entire  surface,  and  the  villi  were  considerably  branched.  The  embryos  of 
this  stage  vary  in  length,  but  measure  about  2.0  mm.  The  medullary  ridges  are 


FIG.  67. — HUMAN  EMBRYO  AT  THE  BEGINNING  OF  THE  THIRD  WEEK. — EIGHTH  STAGE. 
All,  Allantois.     Am,  Amnion.    -br,  Branchial  region.     H,  Head.     Hr,  Heart.     Yk,  Yolk-sac. 

very  characteristic,  rising  high  above  the  yolk-sac  and  enclosing  a  deep  medullary 
groove  between  them.  During  this  stage  the  formation  of  the  segments  is  progress- 
ing. Thus  one  of  the  embryos  described  had  seven  segments. 

Sixth  Stage. — Medullary  Tube:  In  this  stage  the  medullary  groove  is  partly 
closed  and  the  heart  is  clearly  differentiated.  It  must  be  remembered  that  the 
closure  of  the  medullary  groove  progresses  slowly  and  is  not  completed  until  the 
ninth  or  tenth  stage.  The  embryo  measures  from  2.2  to  2.5  mm.  in  length. 
The  head  projects  well  in  front  of  the  yolk.  The  primitive  segments  are  partly 
developed.  In  one  case  seven,  in  another  thirteen,  were  found  to  have  been 
formed.  The  caudal  end  of  the  embryo  also  projects  beyond  the  yolk,  but  less 
than  does  the  head  (compare  Fig.  83).  The  auditory  imagination  is  probably  not 
yet  formed.  There  are  no  gill-clefts  showing  externally. 


THE  CLASSIFICATION  OF  THE  EARLY  STAGES.  121 

Seventh   Stage. — One  Gill-cleft  Showing  Externally:      Not  known  by  observation. 

Eighth  Stage. — Two  Gill-clefts  Showing  Externally:  Several  embryos  in  this  stage 
have  been  found  and  some  of  them  accurately  studied.  They  usually  have  a  re- 
markable bend  in  the  back  (Fig.  67),  which  imparts  to  the  embryo  a  very  singular 
appearance.  Nothing  similar  to  this  bend  or  dorsal  flexure  has  been  observed  in 
any  other  embryos.  It  has  been  held  by  His  and  others  to  be  a  normal  condition, 
and  not  the  accidental  result  of  a  mechanical  strain  exerted  by  the  yolk-sac.  If 
the  condition  is  normal,  it  must  exist  for  only  a  very  brief  period,  as  it  is  not 
encountered  in  older  or  younger  stages.  We  may  suppose  if  it  is  normal  that  the 
change  from  the  concave  to  the  convex  position  of  the  embryo,  as  found  in  the 
next  stage,  is  very  abrupt.  The  head  of  the  embryo  (Fig.  67)  shows  the  character- 
istic head-bend,  and  the  tail  end  of  the  embryo  is  also  bent  over  ventralward.  The 
heart  is  large  and  very  protuberant.  It  is  bent  so  that  we  can  clearly  distinguish 
the  auricular,  ventricular,  and  aortic  limbs.  It  shows  distinctly  its  inner  endothelial 
portion  and  outer  mesoderm.  The  yolk-sac  extends  from  the  heart  backward  to 
where  the  body  of  the  embryo  turns  to  make  the  dorsal  flexure.  Between  the 
heart  and  the  head  the  oral  invagination  has  been  formed,  but  is  still  separated  by 
the  oral  plate  from  the  entodermic  canal.  Above  the  heart  on  either  side  is  an 
open  invagination  of  the  ectoderm,  the  anlage  of  the  so-called  otocyst,  which  in  its- 
turn  is  the  anlage  of  the  epithelial  labyrinth  of  the  adult  ear.  In  one  embryo  of 
this  stage  there  were  found  twenty-nine  primitive  segments. 

Ninth  Stage. — Three  Gill-clefts  Showing  Externally:  This  is,  on  the  whole,  the 
best  known  of  the  early  stages  of  human  development.  The  embryos  described  as 
belonging  to  it  vary  from  2.6  to  4.2  mm.  in  length.  In  one  of  them,  in  which 
the  embryo  measured  3.2  mm.,  the  chorionic  vesicle  measured  n  by  14  mm.,  and 
its  supposed  age  was  from  twenty  to  twenty-one  days.  The  general  shape  of  these 
embryos  is  indicated  -by  figure  89. 

The  head  is  bent  down  and  the  back  is  very  convex.  In  figure  89  the  tail 
is  rolled  up  and  turned  to  the  left.  Usually,  however,  the  tail  turns  to  the  right 
and  the  head  is  twisted  to  the  left,  so  that  the  long  axis  of  the  body  describes  a 
large  segment  of  a  spiral  revolution;  the  spiral  form  is  marked  in  embryos  a  little 
older. 

Tenth  Stage. — Four  Gill-clefts  Showing  Externally:  The  internal  gill-pouches 
reach  the  ectoderm,  and  for  each  there  arises  a  corresponding  external  depression — 
that  of  the  fourth  arch  is  often  indistinct;  hence  this  stage  is  more  easily  recog- 
nized by  the  beginning  of  the  limb-buds.  A  good  embryo  near  the  end  of  this 
stage  has  been  carefully  studied  by  Broman. 

Eleventh  Stage. — Open  Cervical  Sinus:  The  cervical  sinus  is  formed  by  the  invagi- 
nation of  the  ectodermal  area  of  the  fourth  and  fifth  and  later  also  of  the  third 
gill-arches.  The  deep  depression  thus  formed  lasts  for  some  time,  but  closes  over 
ultimately  (embryos  of  10  mm.).  The  eleventh  stage  comprises  a  relatively  long 
period. 


122 


THE  HUMAN  EMBRYO. 


Hypothetical  Development  of  the  Blastodermic  Vesicle  in  Primates. 

As  there  exist  no  direct  observations  on  the  earliest  stages  of  man,  we  can  only 
surmise  what  those  stages  may  be.  It  is  evident  that  there  is  a  very  precocious 
development  of  the  mesoderm,  of  the  extra-embryonic  ccelom,  of  the  amnion,  and 
of  the  trophoderm,  because  these  four  features  are  found  very  marked  in  the  ear- 
liest known  stages  alike  of  man,  apes,  and  monkeys.  There  are  certain  rodents 
and  insectivora  in  which  these  same  peculiarities  occur  more  or  less  emphasized 
in  the  earliest  stages  of  which  we  possess  knowledge.  If  we  utilize  these  data  as 
a  basis,  we  can  reconstruct  the  following  hypothetical  scheme  of  the  earliest  stages 
in  man. 

The  accompanying  diagrams  (Figs.  68  and  69)  represent  three  successive  purely 
hypothetical  stages  of  the  human  ovum.  They  are  all  conceived  to  represent  longi- 


Am.c. ' 


Ent. 


FIG.  68. — Two  DIAGRAMS  TO  ILLUSTRATE  THE  HYPOTHETICAL  EARLY  DEVELOPMENT  OF  PRIMATES. 
Am.c,  Amniotic  cavity.     Cae,  Ccelom.     EC,  Ectoderm,  in  B,  bearing  the  anlages  of  villi.     Ent,  Entoderm.     Mes', 
Somatic  mesoderm.     Mes.",  Splanchnic  mesoderm.     Tro,  Trophoderm. 

tudinal  sections.  In  the  first  stage  the  ectoderm,  EC,  forms  a  moderate  sized  vesicle 
and  is  already  thickened.  It  should  probably  be  conceived  as  consisting  of  an 
inner  distinctly  cellular  layer  and  an  outer  much  thicker  trophodermic  layer  which, 
is  thickest  over  what  corresponds  to  the  embryonic  region.  This  special  thickening 
is  marked  Tro  in  diagram  A.  The  entoderm,  Ent,  forms  a  small  vesicle  underlying 
the  thickened  portion  of  the  trophoderm.  -The  mesoderm,  Mes,  is  well  advanced  in 
its  development  and  already  contains  the  large  extra-embryonic  coelom,  Coe,  and  is 
therefore  divided  into  one  layer  which  surrounds  the  entoderm,  and  a  second  layer 
which  underlies  the  ectoderm.  In  other  words,  the  splanchnopleure  and  somatopleure 
are  already  differentiated.  In  the  next  stage  (Fig.  68,  B)  there  has  been  a  growth, 
the  ovum  has  become  larger,  the  trophoderm  has  increased  in  thickness,  and  in 
the  mass  of  thickened  ectoderm  overlying  the  yolk-sac  there  has  appeared  a  cavity 
— the  future  amniotic  cavity — which  is,  of  course,  entirely  surrounded  by  ectoderm. 
The  portion  of  the  ectoderm  on  the  under  side  of  this  cavity  consists  of  a  single 
layer  of  cells  which  by  assuming  a  cylindrical  form  constitutes  the  thickened  area- 


THE  BLAST ODERMIC  VESICLE  IN  PRIMATES.  123 

which  we  can  identify  as  the  embryonic  shield  (compare  Fig.  13  and  Fig.  68,  B). 
The  solid  mass  of  ectoderm  above  the  amniotic  cavity  is  later  to  form  a  part  of 
the  amnion  and  part  of  the  chorion.  •  At  the  posterior  end  of  the  embryo  there 
appears  a  considerable  accumulation  of  mesoderm  (Fig.  69,  b.s},  which  is  the  an- 
lage  of  the  body-stalk.  Into  this  the  entoderm  has  grown  in  the  form  of  a  cylin- 
drical tubular  prolongation,  the  anlage  oi  the  allantois.  As  a  consequence  of  the 
growth  of  the  trophoderm  and  of  the  formation  of  the  amniotic  cavity,  the  embryo 
or  embryonic  shield,  Emb,  together  with  the  yolk-sac,  Yk,  attached  to  it,  has  been 
forced  down  into  the  interior  of  the  chorionic  vesicle.  This  phenomenon  is  very 
marked  in  certain  rodents  and  leads  to  the  so-called  inversion  of  the  germ-layers. 
In  the  next  stage  the  amnion  is  formed.  This  is  accomplished  by  the  penetration 
of  the  mesoderm  with  accompanying  extension  of  the  extra-embryonic  ccelom  into 


Ent 


'    FIG.  69. — DIAGRAM  OF  AN  EARLY  STAGE  OF  A  PRIMATE  EMBRYO. 

All,   Allantois.     Am,  Amnion.     b.s,   Body-stalk.     Cho,   Chorion.     Emb,   Embryo.     Ent,   Entoderm.     In,   Ento- 
dermal  cavity  of  embryo.     Vi,  Villi  of  chorion.     Yk,  Yolk-sac. 

the  mass  of  the  ectoderm  overlying  the  amniotic  cavity  (compare  Figs.  68,  B,  and 
69)  until  the  condition  shown  in  figure  69  is  brought  about.  This  stage  is  known 
by  observation  (compare  Fig.  80).  The  amnion,  Am,  is  now  completely  separated 
from  the  chorion,  Cho,  which  forms  a  relatively  large  vesicle  and  consists  of  a 
thin  layer  of  mesoderm,  and  a  very  thick  layer  of  ectoderm,  which  has  an  inner 
cellular  stratum  and  an  outer  very  much  thicker  trophodermic  stratum.  The 
trophoderm  is  now  very  much  altered  by  the  appearance  of  numerous  spaces  or 
channels  in  it  which  develop  so  that  each  of  these  spaces  ends  blindly  toward  the 
interior  of  the  chorion,  buf  many  of  them  are  open  upon  the  surface  of  the  tropho- 
derm. As  the  ovum  at  this  stage  is  already  embedded  in  the  uterine  mucosa,  the 
'channels  in  the  trophoderm  can  receive  maternal  blood,  and  such  is  their  original 


124  THE  HUMAN  EMBRYO. 

function.  The  embryo  and  yolk-sac,  as  compared  with  the  chorionic  vesicle,  are 
very  small  in  size.  The  body-stalk,  b.s,  is  well  developed  and  contains  a  well- 
marked  allantoic  anlage,  All,  formed  by  the  entoderm.  The  embryo  includes  as 
yet  very  little,  if  any,  mesoderm.  Probably  a  neurenteric  canal  exists  at  this  stage. 
During  the  transition  of  stage  B  (Fig.  68)  to  stage  C  (Fig.  69),  the  blood-vessels 
appear  in  the  mesoderm  of  the  yolk-sac. 

Relations  of  the  Embryo  to  the  Uterus:   the  Two  Stages. 

The  Two  Stages. — During  the  first  half  or  perhaps  five  months  of  pregnancy 
the  decidua  reflexa  is  present.  This  period  is  called  the  first  stage,  to  distinguish 
it  from  the  remaining  period,  or  second  stage,  during  which  there  is  no  decidua 
reflexa.  The  reflexa  during  the  first  stage  grows  very  thin  and  at  the  same  time 
degenerates.  It  is  finally  resorbed.  The  exact  date  of  its  disappearance  is  not 
known,  but  falls  somewhere  in  the  fifth  month.  During  the  first  stage  the  chorion 
laeve  is  in  contact  with  the  decidua  reflexa,  during  the  second  stage  with  the 
decidua  vera.  On  pages  343  and  345  a  typical  uterus  of  each  stage  is  described. 

The  First  Stage. — The  study  of  young  human  ova  and  of  early  stages  of 
various  primates  leads  us  to  conceive  that  the  ovum  first  implants  itself  in  the 
mucous  membrane  of  the  uterus.  The  conception  "implantation"  is  the  outcome 
of  very  recent  researches.  The  essential  idea  we  have  formed  of  implantation  is 
that  the  trophoderm  of  the  ovum  corrodes  or  digests  the  uterine  tissues  with  which 
it  comes  in  contact,  and  thus  produces  a  cavity  in  which  it  is  lodged  and  where 
it  attaches  itself  intimately  to  the  maternal  tissues.  Owing  to  this  process  the 
ovum  is  at  first  partly  uncovered,  and  this  condition  seems  to  be  permanent  in 
monkeys.  In  man  and  the  apes,  however,  the  uterine  mucosa  grows  over  the 
exposed  portion  of  the  ovum,  forming  a  layer  of  maternal  tissue  which  separates 
the  ovum  from  the  cavity  of  the  uterus.  This  layer  is  the  anlage  of  the  decidua 
reflexa.  As  the  ovum  grows,  the  decidua  reflexa  must  also  expand,  and  we  soon 
reach  a  condition  in  which  the  primitive  relations  of  the  parts  can  be  easily 
followed. 

When  the  uterus  becomes  pregnant,  the  mucous  membrane  of  the  organ 
undergoes  changes  in  structure,  and  it  is  then  commonly  no  longer  termed  the 
mucosa,  but  the  decidua  or  caduca.  The  decidual  membrane  is  histologically 
characterized  by,  first,  modifications  in  the  glands,  the  epithelium  of  which  in  large 
part  degenerates;  second,  the  transformation  of  a  large  number  of  the  connective- 
tissue  cells  into  cells  of  large  size,  which,  on  account  of  their  being  so  extremely 
characteristic,  are  called  the  decidual  cells,  and,  third,  by  a  growth  of  its  blood- 
vessels. 

The  decidual  membrane  of  the  uterus  is  divided  into  three  regions:  first,  the 
decidua  serotina,  the  area  (Fig.  70,  s,s)  to  which  the  ovum  is  attached;  second,  the 
decidua  vera,  comprising  all  the  remaining  portions  of  the  mucosa  forming  part  of 
the  walls  of  the  body  of  the  uterus;  third,  the  decidua  reflexa,  the  arching  dome  of 


RELATIONS  OF  THE  EMBRYO  TO  THE  UTERUS. 


125 


aternal  tissue,  r,r,  which  rises  from  the  walls  of  the  uterus  and  completely  encap- 
sules  the  ovum.  The  arrangement  of  the  parts  is  illustrated  in  figure  70,  which 
represents  a  median  section  of  a  uterus  about  five  weeks  pregnant.  The  whole 
uterus  is  considerably  enlarged.  The  mucous  lining  of  the  uterus  is  very  greatly 
thickened.  The  ovum  is  attached  on  the  dorsal  side  of  the  uterus.  This  is  the 
normal  position.  The  diagrams  so  com- 
monly met  with  which  represent  the  in- 
sertion of  the  ovum  at  other  points  should 
not  be  accepted  by  the  student.  The  reflexa 
rises  around  the  ovum,  completely  covering 
it  in  so  as  to  make  a  closed  bag.  The 
ovum  itself  is  a  sac  known  as  the  chorionic 
vesicle.  The  trophoderm  has  now  quite 
disappeared,  except  so  far  as  it  persists  to 
cover  the  villi.  The  villi  themselves  are 
shaggy  and  more  or  less  branched.  Their 
tips  are  united  either  with  the  surface  of 
the  decidua  serotina  or  with  that  of  the 
decidua  reflexa.  In  the  interior  of  the 
chorion  is  lodged  the  embryo  with  its  yolk- 
sac  and  surrounded  by  the  amnion. 

If  the  walls  of  the  uterus  are  cut 
through  and  simply  reflected,  leaving  the 
bag  of  the  decidua  reflexa  intact,  the  ap- 
pearances will  be  found  essentially  as  in 
figure  71.  The  mucosa  is  enormously  hyper- 
trophied  and  contains  a  great  many  dilated 
irregular  blood-sinuses.  From  the  dorsal 
side  of  the  organ  is  suspended  a  large  ',  Anterior,  &  posterior  surface,  g,  Outer  limit  of 

the  decidua.    s,s,  Limits  of  the  decidua  serotina. 


FIG.  70, — SEMI-DIAGRAMMATIC  OUTLINE  OF  AN 
ANTERO-POSTERIOR  SECTION  OF  A  HUMAN 
UTERUS  CONTAINING  AN  EMBRYO  OF  ABOUT 
FIVE  WEEKS. 


ch,  Chorion,  within  which  is  the  embryo  enclosed 
by  the  amnion,  and  attached  to  the  chorion  by 
the  umbilical  cord;  from  the  cord  hangs  the 
pedunculate  yolk-sac.  r,r,  Decidua  reflexa. 
c,  Cervical  canal. — (After  Allen  Thompson.) 


closed  bag  or  sac,  the  decidua  reflexa, 
D.ref,  nearly  filling  the  cavity  of  the  uterus. 
The  reflexa  presents  in  the  stage  figured 
the  same  general  appearance  as  the  surface 
of  the  uterus.  If  the  reflexa  be  open,  we 
come,  of  course,  upon  the  villous  chorion  of  the  ovum,  and  find,  as  above  stated, 
that  only  the  tips  of  the  villi  are  united  with  the  surface  of  the  reflexa.  In  the 
fresh  state  the  decidua  is  reddish  gray,  spongy  or  pulpy,  soft,  and  moist.  After  the 
fourth  month  it  acquires,  especially  in  the  superficial  layers,  a  duller  brownish  color, 
which  subsequently  becomes  more  marked.  This  coloration  is  due  to  the  decidual 
cells.  During  the  first  two  or  three  months  the  scattered  openings  of  the  uterine 
glands  can  still  be  distinguished  over  the  surface  of  the  serotina  and  vera.  The 
surfaces  themselves  of  the  vera  and  reflexa,  though  somewhat  irregular,  remain 


126 


THE  HUMAN  EMBRYO. 


more  or  less  smooth.  The  inner  surface  of  the  reflexa  is  more  irregular  and 
has  protuberant  parts  united  with  the  tips  of  the  chorionic  villi.  The  surface 
of  the  decidua  serotina,  on  the  contrary,  becomes  very  irregular  during  the 
progress  of  pregnancy,  forming  little  mounds  which  may  become  so  high  as  to 
resemble  columns  or  so  broad  as  to  constitute  septa.  In  later  stages  the  septa 
become  very  well  developed,  attaining  a  height  of  from  5  to  15  mm.  They  are 
irregularly  disposed,  but  subdivide  the  placenta  of  later  stages  into  the  so-called 
cotyledons  (compare  page  362). 


Muse. 


OV. 


-Ovd. 


FIG.  71. — HUMAN  UTERUS,  ABOUT  FORTY  DAYS  ADVANCED  IN  PREGNANCY. 

Muse,  Muscularis.  Dv,  Decidua  vera.  D.ref,  Decidua  reflexa.  Ov,  Ovary.  Ovd,  Oviduct  (Fallopian  tube). 
Lig,  Round  ligament.  Vg,  Vagina.  The  uterus  has  been  opened  by  cutting  through  the  anterior  walls 
and  reflecting  the  sides. — (After  Coste.) 

The  Second  Stage. — The  body-stalk  becomes  converted  into  the  umbilical  cord. 
This  cord  runs  from  the  body  of  the  embryo  to  the  chorion  (Figs.  70  and  87).  It 
is  always  connected  with  that  portion  of  the  chorion  which  is  adjacent  to  the  de- 
cidua serotina.  It  carries  the  arteries  and  veins  from  the  body  of  the  embryo  to 
the  chorion.  From  the  end  of  the  umbilical  cord  the  blood-vessels  branch  out  over 
the  chorion  and  into  the  chorionic  villi.  Thus  the  chorionic  circulation  of  the  em- 
bryo centers  about  the  chorionic  end  of  the  umbilical  cord,  and,  as  this  end  is  in  the 
part  of  the  chorion  overlying  the  decidua  serotina,  we  have  here  established  from 


OVUM  OF  A  MONKEY  IN  THE  SECOND  STAGE.  127 

the  very  start  an  important  factor  in  the  further  differentiation.  From  what  has 
been  said  it  is  evident  that  the  portion  of  the  chorion  underlying  the  decidua  reflexa 
is  more  remote  from  the  center  of  the  embryonic  circulation.  In  the  same  way  we 
find  that  the  decidua  reflexa  is  remote  from  the  blood  supply  in  the  uterus,  and,  as 
a  matter  of  fact,  we  may  observe  that  during  the  second  month  of  pregnancy  the 
blood-vessels,  both  in  the  decidua  reflexa  and  in  the  portion  of  the  chorion  near  it, 
begin  to  disappear  and  ultimately  are  completely  atrophied.  After  this  atrophy  has 
been  accomplished  the  circulation  of  the  chorion  is  restricted  to  that  portion  over- 
lying the  decidua  serotina.  When  the  blood-vessels  of  the  chorion  under  the  de- 
cidua reflexa  abort,  the  villi  also  abort,  so  that  this  part  of  the  chorion  becomes 
smooth,  and  is,  therefore,  called  the  chorion  lave.  Over  the  serotina  the  villi  con- 
tinue to  grow,  hence  the  corresponding  region  of  the  chorion  becomes  known  as  the 
chorion  frondosum.  The  chorion  frondosum  constitutes  the  fetal  portion,  the  de- 
cidua serotina  the  maternal  portion,  of  the  permanent  placenta.  The  maternal  blood 
circulates  in  the  intervillous  spaces,  which  are  bounded  by  fetal  ectoderm.  The 
fetal  blood  circulates  in  the  fetal  blood-vessels  of  the  chorionic  villi.  The  circu- 
latory channels  of  mother  and  fetus  are  always  distinct,  and  no  mingling  of  the 
maternal  and  fetal  blood  is  possible  under  normal  conditions. 

> 

/  Ovum  of  a  Monkey  in  the  Second  Stage.* 

This  embryo  was  obtained  from  a  Semnopithecus  nasicus  in  Borneo  by  Selenka, 
who  has  also  described  an  almost  identical  stage  of  S.  pruinosus.  It  rested  against 
the  wall  of  the  uterus  and  was  uncovered,  there  being  no  decidua  reflexa  developed 
in  monkeys.  It  measured  about  2  mm.  in  its  greatest  diameter.  Figure  72  repre- 
sents a  section  through  the  ovum  and  adjacent  tissues  of  the  uterus.  The  chorionic 
.vesicle  is  very  large,  but  the  embryo,  Sh,  and  yolk-sac,  Yk,  are  relatively  very 
small.  The  chorion  on  one  side  is  quite  smooth;  on  the  opposite  side  it  has  devel- 
oped numerous  outgrowths,  most  of  which  are  formed  exclusively  of  the  ectoderm, 
but  a  few  contain  an  ingrowth  of  mesoderm  in  their  interior.  The  ectoderm  on 
the  side  toward  the  uterus  has  two  layers,  an  inner  cellular  layer  with  relatively 
small  nuclei,  and  an  outer  syncytial  or  trophodermic  layer  with  larger,  nuclei  of 
variable  size.  The  ovum  occupies  a  depression  on  the  surface  of  the  uterus  from 
which  the  uterine  tissues  have  disappeared,  with  the  result  of  breaking  through  the 
walls  of  some  of  the  blood-vessels,  bl.lac,  so  that  now  the  maternal  blood  may  es- 
cape from  these  vessels  into  the  spaces  left  between  the  irregular  outgrowths  and 
the  embryonic  chorion.  We  must  assume  that  the  trophoderm  of  the  embryo  has 
actually  dissolved  away  or  digested  the  tissues  of  the  uterus,  thus  providing  an 
attachment  for  the  ovum,  securing  its  embedding  in  the  wall  of  the  uterus,  and 
establishing  an  opportunity  for  the  maternal  blood  to  flow  into  the  intervillous  spaces. 
In  later  stages  of  the  primates  the  trophoderm  is  very  much  reduced,  and  therefore 

*  Compare  Classification  of  Stages,  p.  IIQ. 


128 


THE  HUMAN  EMBRYO. 


fulfills  its  functions  in  the  very  earliest  stages  by  establishing  these  primitive  condi- 
tions of  blood-supply. 

A  section  of  the  embryo  on  a  larger  scale  is  shown  in  figure  73.  There  ap- 
pears only  the  embryonic  shield,  Sh,  which  is  remarkable  for  its  small  area  and 
great  thickness.  The  yolk-sac  is  also  very  small  and  is  lined  by  a  distinct  layer  of 
entoderm,  Ent.  Above  the  embryonic  shield  is  the  amniotic  cavity,  which  is,  of 
course,  bounded  by  ectoderm  which  is  continuous  with  the  ectoderm  of  the  embry- 
onic shield.  The  amniotic  cavity  has  a  curious  extension  into  the  body-stalk,  b.s, 


Tro. 


Cce.      Mes.       Sh.       Yk. 


Ant.c. 


EC. 


Gl.  bl.lac.  Conn.  Gl. 

FIG.  72. — BLASTODERMIC  VESICLE  OF  A  MONKEY  (SEMNOPITHECUS  NASICUS)  ATTACHED  TO  THE  UTERUS;  VERTICAL 

SECTION. 

Am.c,  Amniotic  cavity,  bl.lac,  Blood-lacuna.  Cce,  Extra-embryonic  coelom.  Conn,  Connective  tissue  of  the 
uterus.  EC,  Ectoderm.  Gl,  Gl,  Uterine  glands.  Mes,  Mesoderm  of  embryonic  chorion.  Sh,  Embryonic 
shield.  Tro,  Trophoblast.  Vi,  Mesodermic  core  of  a  chorionic  villus.  Yk,  Yolk-sac. — {After  E.  Selenka.) 

by  which  the  embryo  is  connected  with  the  chorion.  The  mesoderm  is  chiefly  de- 
veloped over  the  chorion,  as  shown  in  figure  72.  It  is  very  slightly  developed  in 
the  embryo  (Fig.  73,  mes),  but  forms  a  layer  over  the  yolk-sac  and  over  the  am- 
nion,  and  forms  a  considerable  mass  of  tissue  to  constitute  the  body-stalk,  b.s. 

Human  Embryo  in  the  Second  Stage. 

The  embryo  to  be  described  was  investigated  by  H.  Peters.  It  was  found 
attached  to  the  dorsal  wall  of  a  uterus  almost  completely  embedded  in  the  mucosa, 
but  it  was  not  wholly  covered  thereby,  so  that  there  was  no  decidua  reflexa  yet 
present.  A  blood-clot  overlay  what  would  have  been  otherwise  the  exposed  por- 
tion of  the  ovum.  The  trophoderm  formed  an  enormously  thick  layer  of  very 


HUMAN  EMBRYO  IN  THE  SECOND  STAGE. 


129 


irregular  outline  and  contained  many  large  spaces  filled  with  maternal  blood  (Fig. 
74).  The  exact  external  diameter  of  the  ovum  could  not,  therefore,  be  determined. 
It  measured,  however,  approximately  2.4  mm.  by  1.2  mm.  The  internal  diameter 
of  the  chorionic  vesicle  was  about  1.6  by  o .  8  mm.  The  trophoderm  is  every- 
where intimately  united  with  the  uterine  tissue.  The  embryo,  Sh,  is  represented 
by  an  embryonic  shield  consisting  of  cylinder  cells.  It  is  small  and  lies  on  the 
side  of  the  ovum  away  from  the  cavity  of  the  uterus.  It  rests  upon  the  small 
yolk-sac,  Yk,  and  is  overlain  by  the  amniotic  cavity,  Am.  c,  which  is  bounded  every- 
where by  ectoderm — on  one  side,  of  course,  that  of  the  embryonic  shield;  on  the 


A.mes. 


Cce. 


Ent. 


FIG.  73. — EMBRYO  OF  THE  PRECEDING  FIGURE  MORE  HIGHLY  MAGNIFIED. 

Am.c,  Amniotic  cavity.  A.ec,  Amniotic  ectoderm.  A.mes,  Amniotic  mesoderm.  b.s,  Body-stalk.  Cce,  Extra- 
embryonic  ccelom.  Ent,  Entoderm.  mes',  Somatic,  mes",  splanchnic,  mesoderm.  Sh,  Embryonic  shield. — 
(After  E.  Selenka.) 

other  the  thin  amniotic  ectoderm  proper.  The  mesoderm  extends  around  the  ovum, 
forming  a  layer  underneath  the  chorionic  ectoderm  over  the  yolk-sac  and  above  the 
amnion.  At  one  point,  close  to  the  embryo  and  yolk-sac,  it  encloses  a  triangular 
space  the  meaning  of  which  is  not  known.  As  indicated  in  the  figure,  the  meso- 
derm was  found  to  have  shrunken  somewhat,  and  the  appearance  of  the  embryo 
and  yolk-sac  also  suggests  a  somewhat  imperfect  preservation,  histologically  speak- 
ing, of  the  tissues.  As  regards  the  condition  of  the  uterus,  the  following  points  may 
be  noted.  In  the  neighborhood  of  the  ovum  the  decidua  vera  had  acquired  a 
thickness  of  about  8  mm.,  while  on  the  opposite  or  anterior  side  it  was  only  from 
5  to  6  mm.  in  diameter.  Only  in  the  immediate  neighborhood  of  the  ovum  could 
there  be  seen  any  differentiation  of  the  mucous  membrane  into  an  upper,  more 
compact  layer,  and  a  deeper,  looser  cavernous  layer.  The  epithelium  of  the  glands 
and  the  tissues  of  the  uterus  were  well  preserved,  except  in  the  immediate  neighbor- 
hood of  the  ovum.  The  picture  produces  the  impression  that  the  ovum,  in  order 
to  secure  a  place  for  itself,  has  completely  destroyed  the  uterine  tissues  with 
which  it  has  been  in  contact,  thus  implanting  itself  in  the  maternal  tissue.  As  a 

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130 


THE  HUMAN  EMBRYO. 


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THE  EMBRYO  OF  A  GIBBON  IN  THE  THIRD  STAGE.  131 

consequence  of  the  destruction  of  the  maternal  tissues  the  walls  of  some  of  the 
blood-vessels  have  been  broken  through,  and  this  has  allowed  the  blood  to  escape 
from  those  vessels  into  the  lacunae  of  the  trophoderm. 

The  trophoderm  of  the  ovum  offers  a  very  complex  picture,  owing  chiefly  to 
the  changes  which  it  is  undergoing.  The  changes  are  due  apparently  to  hyper- 
trophic  degeneration.  The  layer  of  the  chorionic  ectoderm  next  to  the  mesoderm 
retains  more  or  less  evidently  a  cellular  character.  The  remaining  portions  tend  to 
form  a  syncytium  in  which  the  nuclei  become  enlarged  and  the  cell-boundaries 
obliterated,  while  the  protoplasm  of  the  cells  also  changes  in  character  and  becomes 
more  homogeneous  in  texture  and  much  denser.  The  syncytium  disappears  by  re- 
sorption,  and  its  disappearance  causes  the  formation  of  spaces  in  the  trophoderm. 
Many  different  pictures  occur  in  connection  with  these  processes,  for  in  some  places 
the  nuclei  tend  to  gather  in  groups,  in  others  they  disappear;  in  some  instances 
strands  of  degenerative  material  are  left,  while  nearby  some  of  the  trophoderm  may 
retain  its  more  primitive  appearance  and  be  but  slightly  altered.  Finally,  it  should 
be  noted  that  at  various  points  the  chorionic  mesoderm  is  growing  out  into  the 
trophoderm.  Each  of  these  mesodermic  outgrowths  is  to  be  interpreted  as  the 
anlage  of  the  central  portion  of  a  chorionic  villus,  and  out  of  the  neighboring 
chorionic  ectoderm  will  be  differentiated  the  ectodermal  covering  of  the  villus.  It 
seems,  from  a  comparison  of  later  stages,  that  the  trophoderm  degeneration  never 
goes  so  far  as  to  leave  any  of  the  chorionic  villi  without  an  ectodermal  covering. 
But  this  covering  varies  extremely  in  its  exact  character  as  we  find  it  in  later 
stages,  even  in  adjacent  parts  of  the  same  villus,  for  it  may  be  either  a  single  layer 
of  cells  or  a  layer  of  cells  covered  by  a  thin  coat  of  syncytium  or  merely  a  syncytial 
layer  (compare  page  354).  The  disappearance  of  all  of  the  trophoderm,  except 
so  much  as  remains  to  share  in  forming  the  ectodermal  covering  'of  the  villi,  pro- 
duces the  so-called  intervillous  spaces  of  later  stages,  in  which,  as  above  stated, 
maternal  blood  circulates. 

An  ovum  in  situ  slightly  more  advanced  than  Peters's  has  been  described  care- 
fully by  Herzog.  The  specimen  is  now  in  the  Harvard  Embryological  Collection, 
Series  1500.  It  differs  from  Peters's  ovum  in  having  a  prolongation  of  the  entoderm 
into  the  body-stalk  to  make  the  anlage  of  the  allantois. 

The  Embryo  of  a  Gibbon  in  the  Third  Stage. 

The  embryo  to  be  described  was  obtained  from  a  female  Hylobaies  concolor  in 
Borneo  by  Selenka.  It  is  a  little  more  advanced  than  Frassi's  human  embryo,  men- 
tioned on  page  119.  It  still  had  traces  of  the  primitive  streak,  at  the  anterior  end 
of  which  was  an  open  neurenteric  canal.  The  medullary  plate  was  partially  differen- 
tiated from  the  embryonic  shield.  It  was  thoroughly  studied  by  Selenka,  and  is 
one  of  the  best  known  very  early  ova  of  any  primate.  The  entire  ovum  is  repre- 
sented in  figure  75.  The  figure  was  reconstructed  from  the  sections.  It  shows  the 
chorionic  membrane  studded  with  villi.  The  diameter  of  the  chorion  was  about  8.5 


132 


THE  HUMAN  EMBRYO. 


mm.  The  number  of  villi  was  about  one  hundred,  of  which  some  seventy  are 
clustered  about  the  region  where  the  embryo  was  found.  The  others  are  scattered 
over  the  surface  of  the  membrane.  They  are  considerably  branched.  Each  one  is 
.covered  by  ectoderm  which  consists  of  two  layers,  an  inner  distinctly  cellular,  and 
an  outer  one  in  which  the  cell-boundaries  are  indistinct  and  which  is  known,  there- 
fore, as  a  syncytium  and  represents  the  remains  of  the  original  trophoderm.  Each 


Am. 


Yk. 


Cho 


FIG.  75. — EMBRYO  OF  A  GIBBON  (HYLOBATES  CONCOLOR)  IN  THE  THIRD  STAGE. 
Am,  Amnion.     Yk,  Yolk-sac.     Cho,  Chorion.     Vi,  Villi. — (After  E.  Selenka.) 


villus  contains  a  core  of  mesodermic  tissue.  The  chorionic  membrane  is  repre- 
sented as  open  in  order  to  show  the  size  and  position  of  the  yolk-sac,  Yk,  and  of 
the  amnion,  Am,  which  encloses  the  embryo  as  it  rests  upon  the  yolk-sac.  The 
embryo  itself  is  not  shown  in  the  illustration.  Both  the  yolk-sac  and  the  amnion 
are,  of  course,  covered  by  a  layer  of  mesoderm.  The  entire  space  between  these 
two  inner  structures  and  the  chorion  corresponds  to  the  extra-embryonic  coelom,  the 
very  precocious  and  enormous  development  of  which  is  a  special  characteristic  of 


THE  EMBRYO  OF  A  GIBBON  IN  THE  THIRD  STAGE. 


133 


primates,  including  man,  and  is  not  at  present  known  to  be  paralleled  by  the  con- 
ditions in  the  early  stages  in  any  other  mammals. 

A  side  view  of  the  embryo  on  a  larger  scale  is  represented  in  figure  76.  The 
embryo  is  connected  with  the  chorion  by  a  well-marked  body-stalk,  b.s,  is  covered 
by  the  arching  amnion,  Am,  and  rests  upon  the  yolk-sac,  which  in  comparison  to 
the  chorionic  sac  seems  very  small.  The  yolk-sac,  Yk,  already  has  developed  from 
it  a  network  of  blood-vessels,  Ve,  which  contain  blood-corpuscles  but  have  not  yet 
developed  into  the  embryo  itself.  The  disposition  of  the  vessels  is  best  illustrated  by 
the  section  (Fig.  77).  The  yolk-sac  is,  of  course,  lined  in  its  interior  by  entoderm. 
It  has  formed  already  a  prolongation,  All,  into  the  body-stalk.  This  prolongation 
is  the  anlage  of  the  future  allantois.  Figure  78  represents  a  surface  view  of  the 


Emb.     Ant. 


b.  s. 


Am.      EC.      F. 


Ent: 


Yk. 


All. 


FIG.  76. — EMBRYO  OF  A  GIBBON,  SIDE  VIEW  OF 
THE  EMBRYO  OF  FIGURE  75. 

Emb,  Embryo.  Am,  Amnion.  neu,  Neurenteric 
canal,  b.s,  Body-stalk.  Yk,  Yolk-sac.  Ve, 
Blood-vessels.  All,  Allantois.  —  (After  E. 
Selenka.) 


FIG.  77. — TRANSVERSE  SECTION  OF  THE  EMBRYO 
OF  THE  PRECEDING  FIGURE. 

Am,  Amnion.  EC,  Ectoderm.  F,  Dorsal  furrow. 
mes,  Mesoderm.  Ent,  Entoderm.  Ve,  Blood- 
vessel.— (After  E.  Selenka.) 


same  embryo,  or  perhaps  one  should  say,  rather,  of  the  embryonic  shield.  At  the 
posterior  end  there  is  the  short  primitive  streak,  the  anterior  limit  of  which  is 
marked  by  the  opening  of  the  neurenteric  canal,  neu,  which  passes  obliquely  down- 
ward and  forward,  as  shown  also  in  figure  76.  From  the  end  of  the  neurenteric 
canal  there  extends  forward  a  slight  thickening  of  the  entoderm  which  can  be 
recognized  as  the  anlage  of  the  notochord,  nch.  Figure  77  represents  a  transverse 
section  through  the  region  of  the  notochord.  It  shows  the  amnion,  Am,  arching 
over  the  embryo,  the  thickened  ectoderm  of  the  embryonic  shield,  and  the  anlage 
of  the  notochord.  The  mesoderm,  mes,  of  the  embryo  no  longer  extends  across 
the  median  line  and  is  without  any  ccelom.  At  the  edge  of  the  embryo  the  meso- 
derm splits  and  one  layer  passes  over  on  to  the  amnion,  the  other  on  to  the  yolk- 
sac.  In  the  wall  of  the  yolk-sac,  D,  one  can  easily  distinguish  a  layer  of  the  ento- 
derm, Ent,  and  also  in  the  mesodermic  portion  the  young  blood-vessels,  Ve.  Com- 
parison with  a  section  of  a  somewhat  older  embryo  of  another  gibbon,  Hylobates 


134 


THE  HUMAN  EMBRYO. 


pr.s. 


b.s. 


FIG.  78. — SURFACE  VIEW  OF  THE 
EMBRYONIC  AREA  OF  THE  OVUM 
SHOWN  IN  FIGURE  77. 


Rafflesi,  also  described  by  Selenka,  will  be  found  instructive.  The  relations  are 
here  similar  to  those  shown  in  the  section  just  described,  although  the  stage  is  some- 
what more  advanced,  for  we  see  that  the  amniotic  cavity  is  larger,  that  the  form- 
ation of  the  medullary  groove  has  begun,  that  the 
ccelom  is  beginning  to  appear  in  the  embryonic 
mesoderm,  and  that  the  blood-vessels  of  the  yolk-sac 
have  increased  greatly  in  size.  In  this  embryo  there 
were  traces  of  the  formation  of  three  segments  a  little 
in  front  of  the  neurenteric  canal  which  was  still 
present  and  open.  This  embryo  was  found  to  be 
attached  to  the  wall  of  the  uterus  and  to  be 
enclosed  in  a  decidua  reflexa.  In  later  stages  the 
decidua  reflexa  of  the  gibbon  unites  with  the  decidua 
vera,  and  is  then  lost  completely  by  resorption.  The 
general  character  of  the  ovum  and  its  relations  to 
the  uterus  justify  us  in  the  belief  that  it  is  extremely 
similar  to  the  human  embryo  at  the  same  stage. 

The    Harvard    Embryological    Collection   contains 
one    very    well    preserved   embryo  in  the  third  stage, 

b.s,  Body-stalk,     neu,  Neurenteric  ca- 

nal.  n^Notochord.  pr.s,  Primi-     Senes    825-       It    is    a  little   younger  than  the  gibbon 
tive  streak.  embryo  above  described.     A  monograph  of  this  valu- 

able specimen  is  in  preparation. 

Human  Embryo  in  the  Fourth  Stage  with  the  Medullary  Plate. 

The  general  relations  in  this  stage  have  been  indicated  by  the  diagram  (Fig. 
69).  A  more  exact  idea  of  the  embryonic  structures  may  be  gathered  from  figure 
79,  a  dorsal  view  of  the  embryo,  from  figure  80,  which  represents  a  median  sec- 
tion of  the  embryo  taken  from  a  wax  model  reconstructed  from  the  sections,  and 
figure  81,  a  transverse  section  through  the  neuropore.  The  general  disposition  of 
the  parts  agrees  very  closely  with  the  previous  stage  as  described  for  primates.  The 
embryo  and  yolk-sac  are  very  small  in  comparison  with  the  entire  ovum,  and  they 
are  connected  by  means  of  the  body-stalk,  b.s,  with  the  chorion,  Cho.  The  body- 
stalk  contains  the  entodermal  anlage,  All,  of  the  allantois.  The  embryo  is  covered 
by  the  amnion,  Am,  which  arises  in  front  of  the  head  of  the  embryo,  now  becoming 
marked  off,  and  runs  above  the  embryo  to  join  the  distal  end  of  the  body-stalk. 
The  opening  of  the  yolk-sac,  Yk,  is  about  equal  to  the  length  of  the  embryo.  The 
yolk-sac  is,  of  course,  lined  by  entoderm  and  has  a  thick  layer  of  mesoderm  sup- 
plied already  with  relatively  large  blood-vessels  containing  blood-corpuscles;  the 
vessels  are  developed  chiefly  upon  the  inferior  hemisphere  of  the  yolk-sac.  The 
embryo  measured  i .  54  mm.  in  length.  Its  dorsal  surface  is  represented  in  figure 
79.  This  surface  is  occupied  by  the  very  broad  medullary  plate  of  thickened  ecto- 
derm. Toward  the  middle  of  its  length  the  medullary  plate  is  somewhat  narrower 


HUMAN  EMBRYO  IN  THE  FOURTH  STAGE. 


135 


than  elsewhere.  Along  its  median  line  runs  the  deep,  narrow,  dorsal  groove  which 
at  its  caudal  end  widens  out  and  disappears.  Just  behind  it  is  the  opening  of  the 
relatively  large  neurenteric  canal,  behind  which  again  follows  a  remnant  of  the 
primitive  groove.  A  transverse  section  a  little  in  front  of  the  middle  of  the  em- 
bryo is  shown  in  figure  36.  The  ectoderm,  ek,  is  very  much  thickened  to  con- 
stitute the  medullary  plate;  the  narrow  central  longitudinal  furrow,  /,  mentioned 


Md.gr. 


Neu.  c. 


Cho. 


FIG.  79. — RECONSTRUCTION  OF  A  HUMAN  EMBRYO  i .  54  MM.  LONG.      THE  AMNION  HAS  BEEN  OPENED 

TO   SHOW   THE   DORSAL  SURFACE   OF   THE    EMBRYO. 

Yk,   Yolk-sac.     Am,   Amnion.     Md.gr,   Medullary  groove.     Neu.c,   Neurenteric  canal.     Pr.gr,   Primitive 
groove,     b.s,   Body-stalk.     Cho,   Chorion. — (After  Count  Spec.) 


above  is  very  noticeable.  Outside  of  the  embryo  the  ectoderm  is  reflected  on  to  the 
amnion,  ct,  over  the  back  of  the  embryo.  The  entoderm  is  a  thin  layer  of  cells  in 
the  center  of  which  the  notochordal  band  ch  can  be  distinguished.  In  sections 
near  the  neurenteric  canal  the  notochord  is  better  marked,  being  there  much 
thicker  than  the  remaining  entoderm,  The  mesoderm,  me,  is  a  distinct  layer, 
although,  as  other  sections  show,  it  is  fused  in  the  median  line  of  the  primitive 
streak  behind  the  neurenteric  canal  with  both  ectoderm  and  entoderm.  Although 
the  extra-embryonic  coelom  is  fully  developed,  that  of  the  embryo  is  present  as  a 
small  fissure,  p,  only.  Figure  81  is  a  section  passing  through  the  neurenteric  canal, 
and  shows,  therefore,  the  amnion,  am,  the  thickened  medullary  plate,  e,  of  the  em- 
bryo, and  the  large  yolk-sac,  d.  The  yolk-sac  is  formed,  of  course,  of  splanchno- 


136 


THE  HUMAN  EMBRYO. 


pleure.  The  thickening  of  the  mesodermic  layer  in  the  lower  part  of  the  yolk-sac 
in  order  to  allow  space  for  the  developing  blood-vessels,  b,  b,  b,  is  well  shown  in  the 
figure. 

Eternod  has  studied  an  embryo  in  this  stage.  He  finds  that  the  heart  is 
already  present  underneath  the  slightly  projecting  head.  From  its  anterior  end 
it  sends  out  two  aortic  branches  which  run  on  either  side  near  the  notochord,  pass 
in  a  gentle  curve  around  the  neurenteric  canal,  come  nearer  together  in  the  region 


.<£' 


Ent 


All. 


Yk. 


b.-- 


FIG.  80. — HUMAN  EMBRYO  or  1.54  MM.  MEDIAN  SEC- 
TION FROM  A  WAX  MODEL  RECONSTRUCTED  FROM 
SECTIONS. 

All,  Allantois.  Am,  Amnion.  b.s,  Body-stalk.  Cho, 
Chorion.  EC,  Ectoderm.  Ent,  Entoderm.  mes, 
Mesoderm.  Vi,  Chorionic  villus.  Yk,  Cavity  of 
yolk-sac. — (After  Count  Spec.) 


FIG.  81. — HUMAN  EMBRYO  OF  i  .54  MM. 
Transverse  section  passing  through  the  neu- 
renteric canal  and  yolk-sac,  am,  Amnion. 
ek,  Ectoderm.  ct,  Amniotic  mesoderm. 
g,  Meeting-point  of  somatopleure  and 
splanchnopleure.  df,  Mesoderm  of  yolk- 
sac,  b,  b,  b,  Blood-vessels,  en,  Entoderm. 
n,  Neurenteric  canal,  d,  Cavity  of  yolk- 
sac,  e,  Medullary  plate. — (After  Count 
Spec.) 

of  the  primitive  groove,  and  enter  the  body-stalk,  through  which  they  run  parallel 
to  the  allantois  and  form  ramifications  in  the  chorion.  He  finds  also  two  veins  in 
the  body-stalk  which,  when  they  reach  the  embryo,  unite  to  a  single  median  trunk, 
which  quickly  divides  into  two  vessels  which  run  in  the  mesoderm  of  the  yolk-sac 
near  the  embryo  proper  until  they  reach  the  venous  end  of  the  heart,  into  which  they 
open.  They  each  receive  a  venous  branch  from  the  caudal  side  of  the  yolk-sac. 

Human  Embryo  in  the  Fifth  Stage  with  Open  Medullary  Groove. 

Several  embryos  in  this  stage  have  been  studied.  Two  have  been  studied  by 
W.  His;  one  he  designates  as  "E"  and  the  other  as  "SR"  (Fig.  82).  The 
chorionic  vesicle  of  "E"  measured  8.5  X  5.5  mm.;  of  "SR, "  9X8  mm.  More 


HUMAN  EMBRYO  IN  THE  SIXTH  STAGE. 


137 


satisfactory  is  the  embryo  described  by  Dandy,  a  median  section  of  which  is  given 
in  figure  25.  The  embryo  in  "E"  measured  (?)  2.1  mm.;  in  "SR,"  2.2  mm. 
(Fig.  82).  It  will  be  noticed  at  once  that  .the  condition  is  very  similar  to  that 
shown  in  figure  80,  but  the  embryo  is  somewhat  more  advanced.  The  most 
important  changes  in  the  embryo  at  this  stage  are  its  general  growth,  so  that  it 
rises  above  the  yolk  and  has  both  projecting  head  and  projecting  tail.  The 
medullary  groove  is  very  deep  and  extends  the  entire  length  of  the  embryo.  Toward 
its  caudal  end  it  probably  has  an  open  neurenteric  canal.  The  dorsal  outline  of 


Cho 


Md. 


Am. 


Yk. 


FIG.  82. — HUMAN  EMBRYO  WITH  OPEN  MEDULLARY  GROOVE. 
Am,  Amnion.     b.s,  Body-stalk.     Cho,  Chorion.     Md,  Medullary  folds.     Yk,  Yolk-sac. — (After  W.  His.) 

the  embryo  is  somewhat  concave.  On  the  under  side  of  the  projecting  head, 
between  it  and  the  anterior  limit  of  the  yolk-sac,  the  anlage  o'f  the  heart  has 
appeared,  and  its  cavity  may  be  supposed  to  be  in  connection  with  the  blood- 
vessels of  the  yolk-sac.  The  development  of  segments  has  begun;  Dandy's  embryo 
had  seven.  From  the  under  side  of  the  projecting  tail  end  springs  the  body-stalk, 
to  the  distal  end  of  which  the  chorion  is  attached.  The  chorion  is  completely 
covered  by  short  branching  villi.  The  yolk-sac  has  still  a  very  broad  connection 
with  the  embryo,  and  contains  blood-vessels  throughout  its  entire  extent.  The 
space  between  it  and  the  chorion,  the  extra-embryonic  coelom,  is  very  large. 

Human  Embryo  in  the  Sixth  Stage  with  Medullary  Canal. 

This  stage  does  not  include  the  whole  period  from  the  beginning  to  the  com- 
pletion of  the  closure  of  the  medullary  groove  to  form  the  medullary  canal,  but 
only  the  first  part  of  this  period.  The  best-known  specimen  of  this  stage  was 
described  by  Kollmann.  It  measured  2.2  mm.  in  length  and  had  the  medullary 
groove  open  through  the  anterior  two  thirds  of  its  length,  but  closed  along  the 
caudal  third.  The  embryo  had  thirteen  segments  (Fig.  83).  The  yolk-sac  was 
attached  to  the  embryo  for  a  distance  of  1.5  mm.,  leaving  the  head  to  project 


138 


THE  HUMAN  EMBRYO. 


0.58  mm.  and  the  tail  to  project  0.3  mm.  The  head  is  already  somewhat 
enlarged  and  slightly  bent  over  toward  the  ventral  side.  It  forms  at  least  one 
third  of  the  whole  embryo.  The  dorsal  outline  of  the  embryo  is  concave  in  the 
region  where  the  segments  have  developed.  The  caudal  end  is  slightly  curved  over 
and  is  connected  on  its  under  side  with  the  body-stalk,  Al,  by  which  the  embryo 
is  attached  to  the  chorion.  Between  the  yolk-sac,  Yk.s,  and  the  head,  the  heart, 
Ht,  is  prominent.  By  analogy  with  other  vertebrates  we  assume  that  the  heart- 
tube,  when  it  first  appears  in  man,  is  straight  and  occupies  a  longitudinal  median 


, 

-Am 


Ht 


FIG.  83. — HUMAN  EMBRYO  OF  FROM  THIRTEEN  TO  FOURTEEN  DAYS. 

Am,  Amnion.    5.7,  Seventh  segment.     Md,  Medullary  groove.     Ht,  Heart.     Yk.s,  Yolk-sac.     Al,  Body-stalk 

— (After  J.  Kollmann.) 

position.  In  this  embryo  it  has  already  become  a  relatively  large  organ  and  the 
tube  itself  is  strongly  bent.  No  anlage  of  the  eye  or  ear  was  distinguished.  The 
amnion  was  a  thin,  transparent  membrane  enveloping  the  embryo  quite  closely. 
The  closeness  of  the  amnion  to  the  embryo  was  probably  accidental  (compare  Figs. 
84  and  85).  The  chorion  was  covered  externally  by  branching  villi;  its  diameter, 
including  the  villi,  was  18  mm. 

Another  embryo,  the  position  of  which  in  the  series  of  known  stages  has  long 
been  a  matter  of  dispute,  I  feel,  after  renewed  study,  must  be  assigned  to  a  place 
very  close  to  Kollmann's  embryo  just  described.  The  specimen  in  question  was 
figured  by  Coste  in  his .  monumental  "Atlas  of  Embryology."*  The  embryo  was 
enclosed  in  a  villous  chorion  (Fig.  84)  and  was  provided  with  a  large  vitelline  sac, 

*  The  greatest  difficulty  comes  from  Coste's  statement  as  to  the  magnification  of  his  drawings,  according  to 
which  the  embryo  must  have  been  about  4.4  mm.  long,  or  nearly  double  the  length  which  we  now  know  to  be 
normal  for  embryos  in  the  stage  in  which  this  one  seems,  to  be.  Other  difficulties  arise  because  Coste  has  given 
no  further  description  of  this  embryo  than  that  which  appears  in  the  explanation  of  his  plate.  Neither  that  ex- 
planation nor  the  figures  themselves  afford  any  information  concerning  the  dorsal  side  of  the  embryo  or  as  to 
whether  it  had  a  partially  open  medullary  groove  or  not.  Coste's  figures  indicate  that  thirteen  or  fourteen  seg- 
ments were  visible  externally.  The  shape  of  the  head,  the  size  and  curvature  of  the  heart,  the  form  of  the  tail, 
and  the  concavity  of  the  dorsal  outline  in  the  segmented  region  of  the  embryo  all  indicate  an  extremely  close  resem- 
blance to  Kollmann's  embryo..  As  Coste's  figures  were  all  made  from  fresh  specimens  freehand,  we  shall  prob- 
ably commit  no  error  if  we  assume  that  the  magnification  was  not  correctly  given.  By  making  this  assumption  I 
think  the  difficulties  as  to  placing  Coste's  embryo  vanish. 

Coste's  private  collection  was  said  to  be  at  the  College  of  France,  but  upon  search  this  specimen  could  not 
be  found,  so  that  attempts  to  ascertain  its  actual  length  were  without  result. 


HUMAN  EMBRYO  IN  THE  SIXTH  STAGE. 


139 


FIG.  84. — HUMAN  OVUM,  SAID  TO  BE  FROM  FIFTEEN  TO  EIGHTEEN  t>AYS  OLD.     (Compare  footnote,  page  138.) 
The  chorion  has  been  opened  and  spread  out  to  show  the  embryo  and  its  adnexa.     Al,  Body-stalk  containing  the 
allantoic  diverticulum.     Am,  Amnion  surrounding  the  embryo.     Vi,  Yolk-sac.     (Ajter  Caste.) 


140 


THE  HUMAN  EMBRYO. 


-Spl. 


Vi,  having  a  very  broad  connection  with  the  embryo  and  covered  with  a  network 
of  vessels,  in  which  was  a  fluid  not  yet  red.  A  thick  body-stalk,  Al,  can  be  seen 
running  from  the  under  side  of  the  embryo's  tail  to  the  chorion;  from  the  anterior 
side  of  the  stalk  springs  the  amnion,  Am,  completely  enclosing  the  embryo.  It  is 

important  to  notice  that  in  this,  as.  in  still  older  embryos, 
the  disposition  of  the  amnion  is  essentially  the  same  as  in 
the  earliest  stages;  the  line  of  attachment  of  the  amnion 
is  down  the  sides  of  the  allantois  and  around  the  embryo 
about  on  a  line  with  the  top  of  the  yolk.  As  regards  the 
embryo,  it  is  drawn  slightly  canted  on  to  its  left  side;  its 
back  is  concave;  the  head  end  is  thickest;  behind  and 
below  it  can  be  seen  the  heart,  already  a  bent  tube, 
shining  through;  and  on  the  dorsal  side,  the  light-looking 
oesophagus  is  distinguishable;  in  the  figure  a  wedge-shaped 
shadow  intervenes  between  the  straight  oesophagus  and  the 
bent  heart;  the  heart  causes  a  conspicuous  bulging  of  the 
body  between  the  head  and  the  yolk-sac;  the  caudal  ex- 
tremity is  thick  and  rounded  and  curves  upward.  Figure 
85  is  a  ventral  view  of  the  same  embryo  after  most  of  the 
yolk-sac  has  been  cut  off;  its  walls,  Spl  (splanchnopleure), 
are  seen  to  pass  over  without  any  break  into  those  of  the 
intestinal  cavity.  In  the  central  line  the  notochord,  s, 

can    be    perceived    through    the    translucent    dorsal  wall  of 

FIG.  85.  —  EMBRYO  OF  FIGURE      ,,        .  .      ,  .          .      .      a      ,     ,  ,       .,  ,         , 

0  „  the    intestinal    cavity;    it    is    flanked   on  each   side  by   the 

84,  SEPARATED  FROM  THE  J  ' 

YOLK-SAC    AND    VIEWED     row  of  square  segments.      Behind,  we  see  the  large  body- 

FROM  THE  UNDER  SIDE.        stalk,    Al,   and    in    front    the    tubular    heart,    Ht,   with   a 

Am,  Amnion.  Hi,  Heart.  Spl,     decided   flexure   to   the   right   of   the   embryo;   the  anterior 

Splanchnopleure  extending  j      £    .1        u  i  -^uj  12  a 

beyond  the  embryo  to  form  end  of  the  heart  makes  an  opposite  bend,  separating  off  a 
the  yolk-sac,  s,  Noto-  limb  which  becomes  the  bulbus  aorta.  The  chorion  con- 
chord  with  a  row  of  sjsts  of  ^wo  layers,  one  of  which  forms  the  uninterrupted 
inner  surface  of  the  chorion,  while  the  outer  layer  alone 

* 

forms  the  hollow  villi  (Figs.  84  and  245);  hence,  in  look- 
ing at  the  inside  of  the  chorion,  we  seen  numerous  round 

openings  which  do  not  penetrate  the  inner  layer.  Fortunately,  we  learn  from 
Kolliker,  who  had  an  opportunity  in  1861  to  examine  the  chorion,  that  the  outer 
layer  was  epithelial,  with  cells  of  the  same  character  as  in  the  epithelium  of  older 
vascularized  villi,  and  that  the  inner  layer  consisted  of  developing  connective  tissue, 
and  carried  fine  blood-vessels.  It  thus  appears  that  Coste  was  the  first  to  observe 
the  role  of  the  epithelium  in  the  growth  of  the  villi. 

Human  Embryo  in  the  Seventh  Stage  with  One  Gill-cleft  Showing  Externally. 
No  human  embryo  with  only  one  gill-cleft  showing  externally  is  known. 


side.  Al,  Body-stalk.  (After 

Coste.) 


HUMAN  EMBRYO  IN  THE  EIGHTH  STAGE. 


141 


Human  Embryo  in  the  Eighth  Stage  with  Two  Gill-clefts  Showing  Externally. 

Several  embryos  in  this  stage  have  been  described  and  some  of  them  studied 
anatomically.  Those  which  are  best  preserved  and  which  we  have  best  reason 
to  think  are  normal  present  a  very  singular  appearance,  owing  to  the  deep  bend 
in  the  segmented  region  of  the  body  so  as  to  constitute  at  the  dorsal  outline  of 
the  embryo  at  that  point  a  U-shaped  curve  (Fig.  86). 

This  bend  is  known  as  the  dorsal  flexure.  Embryos  of  earlier  stages  have  an 
indication  of  this  flexure,  as  shown  in  figure  84.  Until  we  have  intermediate 
stages  we  cannot  be  sure  that  the  assumption  which  seems  natural  is  also  correct; 
namely,  that  the  deep  dorsal  flexure  of  figure  86 
is  merely  an  accentuation  of  the  cavity  on 
the  dorsal  side  of  the  embryo  in  earlier  stages. 
In  older  embryos  the  dorsal  flexure  is  normally 
absent  (compare  Fig.  88  and  the  following 
figures).  It  is  possible  that  the  change  from 
the  concave  to  the  convex  position  is  very 
abrupt,  and  it  is  not  improbable  that  the  time 
of  the  occurrence  of  this  change  is  variable. 
The  head  of  the  embryo  and  the  tail  both 
project  far  beyond  the  yolk-sac,  which,  how- 
ever, still  shows  a  broad  attachment  to  the 
embryo.  The  right-angled  head-bend  is  well 
marked  and  the  region  of  the  fore-brain  pro- 
jects 'downward  so  as  to  leave  a  depressed  area  between  the  head  and  the  heart. 
This  depression  corresponds  to  the  position  of  the  oral  cavity.  The  heart  is  large, 
protuberant,  and  considerably  bent,  so  that  we  can  distinguish  its  three  primary 
limbs.  From  the  under  side  of  the  caudal  ^nd  of  the  embryo  springs  the  stout 
body-stalk  by  which  the  embryo  is  united  with  the  villous  chorion.  In  another 
embryo  of  this  stage  there  were  twenty-nine  segments  present.  Above  the  heart 
on  the  side  of  the  pharyngeal  region  two  external  depressions  are  visible  corre- 
sponding to  the  first  two  gill-clefts.  They  are  elongated  in  a  dorso-ventral  direction 
and  are  narrow.  This  position  of  the  amnion  is  well  shown  in  figure  86.  It  arises 
from  the  body-stalk  at  the  side  of  the  embryo  along  the  yolk-sac  and  cardiac  region, 
and  extends  around  the  embryo,  but  is  not  yet  fitted  closely. 

The  anatomy  of  this  stage  is  known  to  us  chiefly  through  the  observations  of 
His  upon  two  embryos  designated  by  him  as  Lg.  and  Sch.  i.  Lg.  measured  2.15 
mm.;  Sch.  i,  2.20  mm.  The  two  embryos  resemble  one  another  closely.  The 
following  description  applies  especially  to  Lg.  The  anatomy  can  be  understood 
from  the  accompanying  figure  87.  The  medullary  tube  extends  the  entire  length  of 
the  embryo  and  is  the  principal  component  of  the  head.  From  the  region  of  the 
fore-brain  has  been  formed  an  outgrowth  to  constitute  the  optic  vesicle,  Op.  At 
the  side  of  the  hind-brain  and  on  the  dorsal  side  of  the  pharynx  is  situated  the 


FIG.  86. — HUMAN  EMBRYO,  2.15  MM.  LONG. — 
(After  W.  His.) 


142 


THE  HUMAN  EMBRYO. 


OP, 


Ao 


anlage  of  the  ear,  Ot,  which  at  this  stage  is  merely  an  open  invagination  of  the 
ectoderm.  The  region  of  the  mid-brain  is  marked  by  the  head-bend,  so  that  the 
axis  of  the  fore-brain  is  approximately  at  right  angles  to  the  axis  of  the  hind-brain. 
Another  consequence  of  the  head-bend  is  that  the  lower  process  of  the  head  is 
brought  very  close  to  the  pericardial  chamber  enclosing  the  heart,  Hi.  Between 
the  head  and  the  pericardial  sac  is  situated  the  oral  invagination  or  future  mouth- 
cavity,  which  is  still  separated  from  the  entodermal 
canal  by  the  oral  plate,  O.pl,  which  consists  merely  of 
a  thin  layer  of  cells  belonging  to  the  ectoderm  and  en- 
toderm  (compare  page  58).  The  pericardial  chamber 
is  large;  in  the  figure  only  the  endothelial  portion  of 
the  heart,  Ht,  is  represented.  Around  this  endothelial 
tube  is  a  second  and  more  bulky  one  from  which  arises 
the  muscular  wall  of  the  heart.  The  volume  of  the 
heart  is,  therefore,  much  greater  than  indicated  by  the 
figure,  hence  the  large  size  of  the  pericardial  chamber. 
On  the  dorsal  side  of  the  heart,  between  it  and  the 
hind-brain,  lies  the  entodermal  canal,  which  is  here  the 
anlage  of  the  pharynx.  It  has  two  diverticula  or  gill- 
pouches  which  are  not  indicated  in  the  figure.  On 
the  side  toward  the  mouth  the  endothelial  part  is 
continued  beyond  the  pericardial  chamber  and  gives  off 

two    vessels    on    each    side,    the    first   and   second  aortic 
FIG.  87. — RECONSTRUCTION  OF  THE  . 

ANATOMY   OF   THE    EMBRYO     arches,  which  pass  around  the  pharynx  and  unite  again 

SHOWN  IN  FIGURE  86.  upon    its    dorsal   side,   and   then,   as   the   aortae,   Ao,   de- 

Op,  Optic  vesicle,   o.pl,  Oral  plate.      scend    along    the    ventral    side    of    the    nervous    system, 
Ht,    Endothelial    heart.    Li.  ,A.  ,,  ,.          ,.  ,  .      , 

, .  soon    uniting    m    the    median    line    to    form    the    single 

Liver.     Om,     Omphalo-mes- 

araic    vein.    Yk,    Yolk-sac,     dorsal  aorta  which   runs  along  nearly  to  the  tail  of  the 
All,    Aiiantoic    diverticuium     embryo,    where    it   forks;    and    its  branches,  passing  one 

formed  by  the%ntoderm    «.„,       Qn    each    sjde    Qf    ^    jntestinal   cana}     enter  the   body-Stalk 
Umbilical     vein.     Ao,    Aorta.  ' 

Ot,  Otocyst.— (After  w.  His.)     and  run  to  the  chorion,  where  they  branch  out.     Behind 

the  pharynx  the  entodermal  canal  merges  into  the  cavity 

of  the  yolk-sac,  Yk,  and  then  beyond  the  yolk-sac  extends  again  into  the  tail  of 
the  embryo,  forming  an  expansion  there  which  is  known  as  the  cloaca.  From 
the  under  side  of  the  cloaca  runs  out  the  allantoic  diverticuium,  All,  which  extends 
as  a  narrow  tube  of  entoderm  through  the  allantoic  stalk  to  the  level  of  the  chorion, 
where  it  ends  blindly.  The  pericardial  chamber  on  its  caudal  side  is  bounded  by 
the  septum  transversum,  in  which  we  find  the  anlage  of  the  liver,  Li,  already 
present,  and  through  which,  on  either  side,  the  great  vein  from  the  yolk-sac,  the 
omphalo-mesaraic  or  vitelline  vein,  Om,  passes  to  the  heart.  Of  the  veins  of  the 
embryo  only  the  umbilical,  u.v,  is  shown  in  the  figure.  This  vein  gathers  the 
vessels  from  the  chorion,  passes  through  the  body-stalk,  then  runs  in  the  somato- 


HUMAN  EMBRYO  IN  THE  NINTH  STAGE. 


143 


pleure  of  the  embryo  to  join  the  omphalo-mesaraic  vein  and  enter  the  heart.  In 
the  figure  only  the  general  course  of  the  vein  is  indicated.  The  fact  that  it  is 
situated  in  the  somatopleure  could  not.  well  be  shown. 

Human  Embryo  in  the  Ninth  Stage  with  Three  Gill-clefts  Showing  Externally. 

Our  knowledge  of  this  stage  is  quite  good.  The  described  embryos  vary  in 
length  from  2.6  to  4.2  mm.  The  chorionic  vesicles  are  about  10  mm.  in  diam- 
eter, varying  according  to  the  size  of  the  embryo.  Figures  88  and  89 
represent  two  embryos  of  this  stage,  the  latter  being  the  more  advanced.  The 
back  of  the  embryo  is  normally  (or  at  least  usually)  convex.  The  head  is  bent 


FIG.  88. — HUMAN  EMBRYO  OF  2.6  MM.  LENGTH. — (After  W.  His.) 


to  one  side,  usually  to  the  right,  and  the  tail  to  the  other,  the  whole  embryo  hav- 
ing a  slight  spiral  twist.  The  embryo  has  become  quite  large  in  proportion  to 
the  yolk-sac.  The  three  gill-clefts  are  readily  seen,  the  first  being  the  largest,  the 
third  the  smallest.  The  column  of  tissue  between  the  first  cleft  and  the  mouth  is 
the  mandibular  process.  Between  it  and  the  fore-brain  lies  the  shorter  rounded 
maxillary  process.  The  segments  are  clearly  marked  externally  along  the  back 
(Fig.  89).  The  origin  of  the  amnion  is  shown  in  figure  89  also.  The  heart  has 
grown  and  something  of  its  more  complicated  form  is  indicated  in  the  external 
modeling  of  the  embryo.  The  anlage  of  the  future  ear  is  now  a  closed  vesicle  or 
otocyst  (Fig.  90,  of).  From  the  region  over  the  heart  almost  to  the  caudal 
extremity  the  segments  of  the  body  are  distinctly  marked  externally. 

The  general  anatomy  of  this  stage  will  be  understood  by  the  aid  of  the  accom- 
panying figures  90  to  93,  which  are  all  reconstructions  from  sections.  The  position 
of  the  notochord,  Ch,  is  indicated  by  a  line  (Fig.  91).  The  pharynx  is  large  and 


144 


THE  HUMAN  EMBRYO. 


wide.  It  has  three  lateral  outgrowths  on  each  side,  i,  2,  3,  the  gill-pouches.  In 
front  and  near  the  cephalic  end  of  the  notochord  there  is  a  small  median  out- 
growth, the  anlage  of  the  hypophysis,  Hy.  Toward  the  neck-bend  the  pharynx 
becomes  narrower  and  passes  over  into  the  small  entodermal  tube,  from  which  we 
can  detect  the  outgrowth,  Lu,  which  represents  the  commencing  formation  of  the 
lungs.  This  narrow  tube  leads  to  the  space  above  the  yolk-sac,  Yk.s.  Just 


FIG.  89. — HUMAN  EMBRYO  4.2  MM. 
Yks,   Yolk-sac.     Am,   Amnion.     All, 
Body-stalk.— (After  W,  His.) 


Car. 


FIG.  90. — RECONSTRUCTION  or  THE 
ANATOMY  OF  THE  EMBRYO  OF  2.6 
MM.  IN  FIGURE  88. 

A,  Aortic  limb  of  heart.  All,  Body-stalk. 
Ao,  Dorsal  aorta.  Au,  Umbilical 
arteries.  Car,  Posterior  cardinal 
vein.  Jg,  Anterior  cardinal  vein. 
Om,  Omphalo-mesaraic  vein,  op, 
Optic,  vesicle.  ot,  Otocyst.  Vh, 
right  umbilical  vein. — (After  W. 
His.) 


where  it  passes  into  the  yolk-sac  the  entoderm  has  formed  the  rudiment  of  the 
liver,  Li.  Figure  55  gives  a  view  of  the  anterior  wall  of  the  pharynx  of  another 
embryo.  In  front  is  the  large  opening  of  the  mouth,  M,  the  oral  plate  between 
the  mouth-cavity  and  the  entodermal  canal  having  disappeared.  This  embryo 
being  a  little  older,  the  traces  of  the  four  gill-clefts  can  already  be  seen,  and  there 
are  four  entodermal  gill-pouches.  The  aortic  vessels  are  indicated  by  dotted  lines. 
The  cardiac  aorta  reaches  the  pharynx  between  the  bases  of  the  second  and  third 
gill-arches,  and  divides  into  two  branches,  one  on  each  side.  The  anterior  branch 


HUMAN  EMBRYO  OF  THE  NINTH  STAGE. 


145 


forks  and  runs  through  the  first  and  second  arches.  The  posterior  branch  forks, 
one  fork  going  to  the  third,  and  the  other,  after  again  forking,  supplies  the  fourth 
and  fifth  branchial  arches.  This  arrangement  of  the  aortic  branches  is  typical. 
Between  the  bases  of  the  first  and  second  arches  is  a  small  protuberance  which  is 
the  anlage  of  the  tongue  and  is  named  by  His  the  tuberculum  impar.  Studies  of 
the  sections  demonstrate  that  the  cavity  of  the  abdominal  region  (splanchnocele) 
has  on  each  side  of  its  dorsal  surface  a  longitudinal  ridge,  the  commencement  of 
the  Wolffian  body.  The  ridge  already  contains  traces  of  the  canals  of  the  Wolffian 


FIG.  91. — OUTLINE  OF  THE  ENTODERMAL  CANAL  OF 
A  HUMAN  EMBRYO  OF  4 . 2  MM. 

Hy,  Hypophysis,  i,  2,  3,  Lines  marking  the 
position  of  the  pharyngeal  gill-pouches.  Lu, 
Lungs.  Li,  Liver.  Yks,  Yolk-sac.  Al,  Allan- 
tois.  W,  Wolffian  duct.  Ch,  notochord. — 
(After  W.  His.) 


Op 


-V 


FIG.  92. — RECONSTRUCTION  OF  THE  ANATOMY  OF  A 
HUMAN  EMBRYO,  3 . 2  MM.  LONG,  SHOWING  THE 
ANTERIOR  END  VIEWED  FROM  THE  VENTRAL  SIDE. 

Op,  Optic  vesicle.  Ht,  Heart.  Li,  Liver.  V,  Allan- 
toic  vein.  Au,  Auricle  of  the  heart,  i,  2,  3,  4, 
Aortic  arches. 


body.  Of  especial  interest  is  the  arrangement  of  the  circulatory  apparatus  (Figs. 
88  and  92).  In  the  first  figure  the  arteries  are  shaded  dark;  the  heart  is  an 
S-shaped  tube  which  is  really  double,  consisting  of  an  inner  endothelial  tube  con- 
tinuous with  the  arteries  and  veins  at  either  end  of  the  heart,  and  an  outer  meso- 
dermic  tube  which  is  confined  to  the  heart  and  is  unconnected  with  the  blood-vessels. 
The  venous  end  of  the  heart  lies  near  the  yolk-sac.  It  is  convex  toward  the  head. 
The  arterial  end  of  the  heart  is  convex  toward  the  tail.  When  viewed  from  the 
ventral  side,  the  venous  process  .of  the  heart  (Fig.  92,  Au}  is  seen  on  the  left  and 
the  arterial  process,  Ht,  is  seen  on  the  right.  The  heart  is  continued  forward  by 
the  large  aorta  (Fig.  90,  A),  which  gives  off  five  branches  on  each  side  of  the  neck. 
These  branches  again  unite  on  the  dorsal  side  and  run  backward  to  unite  with 


146 


THE  HUMAN  EMBRYO. 


Ot 


the  fellow-stem,  and  so  form  the  single  median  dorsal  aorta,  Ao,  which  runs  way 
back  and  terminates  in  two  branches,  Au,  which,  curving  round,  pass  out  through 
the  body-stalk  and  supply  the  circulation  of  the  chorion.  The  five  branches  in 
the  neck  are  known  as  the  aortic  arches.  The  column  around  each  branch  con- 
stitutes the  so-called  branchial  arch.  Each  branchial 
arch  is  further  marked  out  by  the  gill-cleft  in 
front  of  it  and  behind  it,  as  shown  in  figure  90. 
The  reconstruction  of  the  third  embryo  in  the  side 
view  (Fig.  93)  affords  further  information  concern- 
ing the  disposition  of  the  heart  and  the  large 
blood-vessels.  The  veins,  as  is  there  shown,  are 
(i)  the  anterior  cardinals,  J,  which  are  often  re- 
ferred to  as  the  jugular  veins,  although  they  are 
not  identical  with  the  jugulars  of  the  adult;  (2)  the 
posterior  cardinals  (compare  Fig.  90,  Car};  the 
posterior  and  anterior  cardinals,  coming  from  the 
caudal  and  cephalic  regions,  respectively,  unite  to 
form  a  single  transverse  stem,  the  common  cardinal, 
D.C  (the  posterior  cardinals  receive  their  blood 
chiefly  from  the  Wolfiian  bodies,  and  later  undergo 
complicated  metamorphoses);  (3)  the  large  umbili- 
cal or  allantoic  veins,  Al.v,  which  pass  up  from 
the  chorion  through  the  body-stalk  into  the  somato- 
pleure  until  at  the  level  of  the  septum  trans- 
versum,  above  the  liver,  Li,  they  empty  into  the 
common  cardinal;  (4)  the  omphalo-mesaraic  or 
vitelline  veins,  om,  which  corne  up  from  the  yolk- 
sac  on  either  side  and  meet  the  common  cardi- 
nals at  the  venous  end  of  the  heart.  This  figure 
also  shows  the  disposition  of  the  aortic  arches  and 
Art,  Allantoic  artery.  Al.v,  Allan-  an  early  stage  of  the  primitive  internal  carotid 


FIG.  93. — RECONSTRUCTION  OF  THE  AN- 
ATOMY OF  THE  HUMAN  EMBRYO  OF 
4 . 2  MM.  SHOWN  IN  FIGURE  89. 

Ot,  Otocyst.  J,  Anterior  cardinal  vein. 
car,  Carotid  artery.  7,  First  aortic 
arch.  Au,  Auricle.  Ven,  Ventricle. 
Li,  Liver,  om,  Omphalo-mesaraic 
vein.  Al,  Allantoic  diverticulum. 


toic  vein.  Am,  Origin  of  the  amnion. 
D.C,  Common  cardinal. — (After  W. 
His.) 


artery,  car.     The  muscular,  but  not  the  endothelial, 
heart  is  represented  in  the  reconstruction. 


Human  Embryo  in  the  Tenth  'Stage  with  Four  Gill-clefts  Showing  Externally. 
Few  embryos  belonging  to  this  stage  have  been  obtained.  The  one  shown  in 
figure  94  was  carefully  studied  and  described  by  W.  His.  Its  probable  age  is 
twenty-three  days.  The  embryo  forms  almost  a  complete  circle,  the  tail  being 
close  to  the  head.  The  limb-buds  have  appeared.  .  The  heart  is  large  and  causes 
a  marked  swelling  of  the  body  beneath  the  branchial  arches,  i,  2,  3,  4,  all  four  of 
which  show  clearly  on  the  surface.  The  entodermal  canal  has  attained  nearly 
the  condition  shown  in  figure  27,  B. 


HUMAN  EMBRYO  IN  THE  ELEVENTH  STAGE. 


147 


Human  Embryo  in  the  Eleventh  Stage  with  the  Cervical  Sinus  in  Formation. 

The  embryo  figured  (Fig.  95)  was  described  by  Mall,  and  one  almost  identical 
has  been  studied  by  H.  Piper.  Its  age  is  probably  twenty-six  days.  At  this 
stage  the  embryo  is  flexed  so  as  to  describe  almost  a  circle,  the  tail  being  almost 
in  contact  with  the  head,  yet  comparison  with  figure  94  reveals  that  the  straightening 
of  the  back  of  the  embryo  has  begun.  Although  the  limbs,  A.I  and  P.I,  have 
increased  in  size,  they  are  still  only  rounded  buds.  The  head,  which  is  bent  to 
the  right,  partly  conceals  the  cardiac  region.  The  nasal  pit,,  Na,  is  a  broad, 


IV 


al 


s.s 


FIG.  94. — HUMAN  EMBRYO  OF  ABOUT  TWENTY- 
THREE  DAYS,  4  .o  MM.  X  15  diams. — (After 
W.  His,  Embryo  a.) 

al,  Anterior  limb-bud.  B.S,  Body-stalk.  Op, 
Optic  vesicle,  pi,  Posterior  limb-bud,  iv, 
Fourth  ventricle,  i,  Mandibular  process. 
2,  Hyoid  arch.  3,  4,  Third  and  fourth  gill- 
arches. 


Cerv.s 


A.I. 


P.I. 

FIG.  95. — HUMAN  EMBRYO  OF  7  .o  MM.     X  8  diams. 

—(After  F.  P.  Mall.) 

A.I,  Anterior  limb.  Cerv.s,  Cervical  sinus.  Li,  Liver. 
Md,  Mandibular  process.  MX,  Maxillary  process. 
Na,  Nasal  pit.  Op,  Eye.  P.I,  Posterior  limb. 
Um.c,  Umbilical  cord.  Yk.s,  Yolk-stalk. 


shallow  fossa.  The  eye,  Op,  consists  of  the  small  optic  vesicle  and  overlying  lens. 
The  maxillary  process,  MX,  is  well  developed.  Behind  the  mandibular  process, 
Md,  is  the  first  cleft,  or  anlage  of  the  external  auditory  meatus.  The  cervical 
sinus,  Cerv.s,  is  in  process  of  development,  but  on  the  left  side  is  not  so  deep  as 
on  the  right  side,  which  is  figured.  The  ventral  ends  of  the  branchial  arches  are 
continuous  with  the  cardiac  region  of  the  body.  As  shown  in  the  figure,  twenty- 
four  segments  are  clearly  marked  externally.  The  large  cardiac  region  fills  out 
the  space  between  the  anterior 'limb,  A.I,  and  the  tip  of  the  head.  The  ventral  sur- 
face of  the  abdomen  is  prolonged  to  form  the  umbilical  cord,  Um.c,  from  which 
projects  the  slender  yolk-stalk,  Yk.s.  The  position  of  the  liver  is  indicated  by  a 
distinct  protuberance  below  the  foreleg. 


148 


THE  HUMAN  EMBRYO. 


Human  Embryos  of  the  Fourth  Week  to  the  Fourth  Month. 

The  following  series  of  illustrations  (Figs.  96-113  inclusive)  are  from  specimens 
in  the  Harvard  Embryological  Collection,  all  normal  or  nearly  so.  To  facilitate 
comparison  figures  97-107  are  uniformly  magnified  five  diameters,  while  figures  108-113 
are  life  size. 

Embryos  of  Four  Weeks,  7.5  to  8.0  mm.— They  are  characterized  especially 
by  the  extreme  development  of  the  neck-bend.  The  fourth  and  fifth  branchial 
arches  are  entirely  buried  in  the  cervical  sinus,  and  the  third  arch  is  turning 
in.  In  other  words,  the  process  of  invagination  of  the  sinus,  though  far  advanced, 


FIG.  96. — HUMAN  OVUM  WITH  EMBRYO  OF  9.4  MM. 
THE  CHORION  HAS  BEEN  PARTLY  REMOVED  TO 
SHOW  THE  EMBRYO.  X  3  diams. — (Minot  Collec- 
tion, 275.) 


FIG.  97. — HUMAN  EMBRYO  OF  9.6  MM. 
SERIES  1001.      X  5  diams. 


is  not  completed.  The  invagination  of  the  ectoderm  to  form  the  lens  is  still  open, 
though  about  to  close.  The  back  of  the  embryo  is  partly  straightened.  The  limb 
buds  are  beginning  to  expand  at  their  distal  ends  to  make  the  anlages  of  the  hands 
and  feet. 

Embryos  of  Twenty-eight  to  Thirty  Days,  8.0  to  10.0  mm. — The  form  of  hu- 
man embryos  at  the  end  of  the  first  month  is  very  variable,  and  it  has  not  been 
possible  hitherto  to  establish  with  certainty  a  typical  normal  shape.  Their  length 
varies  because  the  head  begins  to  rise  with  accompanying  diminution  of  the  neck- 
bend,  hence  the  length  may  be  increased  by  a  change  of  form  without  a  correspond- 
ing growth  of  the  embryo  as  a  whole  or  advance  in  structure.  Figure  96  illustrates 
the  proportions  of  the  embryo,  yolk-sac,  and  chorion  at  this  stage.  Figure  97  shows 
an  embryo  of  9.6  mm.  with  the  yolk-sac  and  stalk.  In  this  specimen  the  oblitera- 


EMBRYOS  OF  THIRTY-ONE  TO  THIRTY -TWO  DAYS. 


149 


tion  of  the  neck-bend,  the  growth  of  the  limbs,  the  narrowness  of  the  opening  of 
the  sinus  cervicalis,  the  elongation  of  the  umbilical  cord,  and  the  expansion  of  the 
hind-brain  are  all  evidences  of  advancing  development  (compare  Fig.  95).  From 
the  distal  end  of  the  umbilical  cord  springs  the  amnion,  beyond  which  there  passes 
out  from  the  cord  the  narrow  stalk  of  the  yolk-sac.  The  cavity  in  the  interior  of 
the  cord  is  a  continuation  of  the  ccelom  of  the  embryo  and  through  it  the  yolk- 
stalk  takes  its  course. 

Figure  98  is  very  instructive,  for  it  represents  an  embryo  which,  although  0.2  mm. 
shorter  than  the  one  shown  in  figure  97,  yet  is  much  more  advanced  in  development, 
as  is  evidenced  strikingly  by  the  enlargement  of  the  whole 
.  head  and  the  elongation  of  the  limbs  and  the  demarcation 
of  the  hand  from  the  rest  of  the  anterior  limb.  The 
orifice  of  the  cervical  sinus  is  narrow.  On  the  ventral 
side  of  the  anterior  limb,  the  body  shows  three  rounded 
eminences  corresponding  to  the  auricle  of  the  heart,  the 
ventricle  of  the  heart,  and  .the  liver. 

Embryos  of  Thirty-one  to  Thirty-two  Days,  10  to  12 
mm. — As  typical  specimens  of  this  stage  we  may  take  two 
embryos,  one  of  10.0  mm.  (Figs.  99  and  100),  the  other 
of  11.5  mm.  (Fig.  101).  Figure  99  shows  the  embryo, 
Series  1000,  the  chorion  and  amnion  having  been  opened; 
the  embryo  lies  somewhat  obliquely  on  its  left  side,  therefore 
figure  100  has  been  added  to  give  a  correct  profile  compara- 
ble with  the  other  figures  of  the  series.  As  compared  with  the  previous  stage  (Fig. 
97),  the  back  has  straightened  out  somewhat,  though  the  lower  end  of  the  body 
is  still  rolled  over.  The  head  has  risen  and  increased  considerably  in  size.  Be- 
tween the  end  of  the  region  of  the  hind-brain  and  the  level  of  the  arm  the  dorsal 
outline  has  become  slightly  concave.  This  concavity  His  designated  the  "Nacken- 
grube."  The  first  gill-cleft,  owing  to  the  completed  closure  of  the  cervical  sinus,  is 
the  only  one  visible  externally.'  It  is  the  anlage  of  the  external  auditory  meatus. 
It  is  separated  from  the  mouth  by  a  prominent  mandibular  arch.  On  the  cephalic 
side  of  the  mouth  the  maxillary  process  has  become  more  prominent,  but  the  two 
portions  of  the  maxilla  do  not  yet  meet  in  the  median  line.  The  primitive  seg- 
ments are  still  marked  externally.  The  limbs  show  indications  of  their  tripartite 
division,  the  fore-limb  being  more  advanced  than  the  hind-limb.  The  division  of 
the  digits  of  the  hand  is  just  indicated.  The  abdomen  -bulges  out,  owing  to  the 
growth  of  the  liver.  There  is  a  true  tail,  which  is  now  near  its  maximum  develop- 
ment. The  umbilical  cord  has  lengthened  and  shows  the  commencement  of  its 
spiral  twisting.  The  amnion  springs  from  the  end  of  the  cord,  leaving  only  a  short 
stretch  of  the  body-stalk  between  the  cord  proper  and  the  chorion.  The  amnion 
envelops  the  embryo  closely.  In  embryos  slightly  older  than  these  the  changes  in 
form  above  mentioned  have  progressed  further.  The  body  is  straighter,  the  head 


FIG.  98. — HUMAN  EMBRYO  OF 
9.4  MM.  SERIES  1005. 
X  5  diams. 


150 


THE  HUMAN  EMBRYO. 


is  larger,  and  has  risen  so  as  to  be  about  at  right  angles  to  the  body.  The  con- 
cavity (Nackengrube)  below  the  hind-brain  in  the  outline  of  the  neck  is  more 
marked.  The  limbs  are  longer,  the  fingers  more  distinct.  Where  the  mandibles 
meet  in  the  median  line,  the  separation  of  lip  and  chin  has  begun. 

Embryos  of  Thirty-six  Days,  14  mm. — The  correlation  of  age  and  size  for  this 
stage  cannot  be  recorded  as  absolute,  but  we  may  say  that  embryos  of  this  length 


FIG.  99. — HUMAN  EMBRYO  OF  ro.o  MM.  WITH  THE  AMNION,  CHORION,  AND  YOLK-SAC.     SERIES  1000.    X  5  diams. 

(Compare  Fig.  100.) 


are  about  five  weeks  old.  The  body  is  now  nearly  straight  (Fig.  102).  The 
lower  limbs  project  beyond  the  outline  of  the  body  in  profile  views.  The  bulging 
of  the  outline  at  the  neck-bend  is  characteristic  of  this  stage,  but  in  the  specimen 
figured  the  protuberance  is  unusually  great.  The  ventral  outline,  owing  to  the  large 
size  of  the  heart  and  liver,  is  very  protuberant,  and  at  this  stage  we  find  that  the 
portion  of  the  umbilical  cord  adjoining  the  embryo  is  greatly  enlarged,  owing  to  the 
distention  of  its  coelom,  so  that  a  large  cavity  is  furnished  in  which  there  are  al- 
ways found,  as  indicated  in  figure  84,  several  coils  of  intestine.  This  protrusion  of 
a  portion  of  the  intestinal  canal,  and  sometimes  even  of  a  small  portion  of  the 


EMBRYOS  OF  THIRTY -EIGHT  DAYS. 


151 


liver,  into  the  extra-embryonic  coelom  of  the  umbilical  cord  is  a  constant  phenom- 
enon. It  begins  at  a  somewhat  earlier  stage  and  continues  for  a  considerable 
period.  This  curious  condition  has  been  observed  in  many  different  kinds  of  mam- 
mals in  the  corresponding  stage.  Later  on,  the  viscera  are  entirely  withdrawn  from 
the  umbilical  cord  and  the  cavity  itself  is  wholly  obliterated.  The  umbilical  cord 
is  a  hollow  prolongation  of  the  body-wall  or  somatopleure  of  the  embryo,  and  the 
amnion  springs  from  its  distal  end.  The  yolk-stalk  is  very  long  and  narrow.  Its 
entodermal  cavity  is  obliterated.  It  is  the  representative  of  the  original  broad  con- 
nection between  the  yolk-sac  and  the  entodermal  cavity  of  the  embryo,  although 
it  is  now  only  a  small  appendage  of  a  loop  of  the  intestine.  It  bears  the  blood- 


FIG.  ioo. — HUMAN  EMBRYO  OF  10.0  MM.    SERIES 
1000.      X  5  diams.     (Compare  Fig.  99.) 


FIG.  101. — HUMAN  EMBRYO  OF  11.5  MM. 

SERIES  1006.      X  5  diams. 


vessels  which  run  from  the  embryo  and  ramify  upon  the  yolk-sac.  On  the  caudal 
side  of  the  umbilical  cord  we  find  the  tissue  of  the  original  body-stalk  in  which 
run  the  allantoic  vein  and  the  two  allantoic  arteries  which  ramify  upon  the 
chorion. 

Embryos  of  Thirty-eight  Days,  16  mm.  in  a  chorionic  vesicle  of  45  by  40  mm.— 
The  age  of  this  specimen  (Fig.  103)  is  known  by  estimate  only.  This  stage  repre- 
sents the  transition  of  the  embryo  to  the  fetus,  because  after  the  fortieth  day  the 
form  is  distinctly  human.  The  head  has  risen  considerably,  and  the  back  has 
straightened  still  more,  the  rectangular  neck-bend  thus  becoming  emphasized.  The 
body  has  become  still  more  protuberant  on  the  ventral  side,  and  in  side  views  the 
arms  reach  to  the  outline  of  the  body.  In  the  anterior  limb  we  note  the  first  indi- 
cations of  the  five  digits  and  of  the  separation  of  the  upper  and  lower  arms.  To 


152 


THE  HUMAN  EMBRYO. 


illustrate  the  variations  in  the  proportions  of  embryos  and  to  show  a  slightly  more 
advanced  stage,  figure  104,  of  a  17.8  mm.  embryo  is  given  and  also  figure  105, 
A,  B,  giving  two  views  of  an  embryo  of  18.1  mm.  All  three  specimens  are  proba- 
bly normal,  for  it  is  known  that  variation  is  much  greater  during  development 
than  in  the  adult,  a  fact  which  is  to  be  explained  in  large  part  by  the  temporary 
accelerations  or  retardations  of  the  development  of  single  organs  or  regions,  which 
are  subsequently  compensated  for. 

Embryos  of  Forty  Days,    19   mm. — The   head   has   risen   far  toward    its   definite 
position,   with   the   result  of  a   very   rapid   apparent   increase   in   the   total   length   of 


FIG.  102. — HUMAN  EMBRYO  OF  14.5  MM.     SERIES  1003.      X  5  diams. 

the  embryo.  The  change  of  position  of  the  head  results  in  bringing  the  mid-brain 
finally  directly  above  the  hind-brain,  'the  embryo  being  conceived  as  having  the  body 
vertical.  During  the  .elevation  of  the  head  the  concavity  (Nackengrube)  at  the  back 
of  the  neck  is  gradually  obliterated.  In  both  head  and  rump  the  external  modeling, 
which  in  earlier  stages  indicated  more  or  less  the  position  of  the  internal  organs, 
has  become  blurred,  and  in  the  next  stage  is  found  to  have  nearly  or  quite  disap- 
peared. The  maxillary  processes  have  met  and  united  in  the  median  line.  The 
anlages  of  the  eyelids  have  developed.  The  concha  of  the  ear  is  indicated.  The 
arm  reaches  beyond  the  heart;  the  fingers  appear  as  separate  outgrowths. 

Embryos  of  Fifty  Days,  21  mm. — The  author  has  a  fair  specimen  which  came 
into  his  possession  with  no  history  whatever,   but  it  agrees  very  closely  with  His's 


EMBRYOS  OF  THIRTY-EIGHT  DAYS. 


153 


FIG.  103. — HUMAN  EMBRYO  OF  16.0  MM. 
SERIES  1128.       X  5  diams. 


FIG.  104. — HUMAN  EMBRYO  OF  17.8  MM. 
SERIES  839.     X  5  diams. 


A  B 

FIG.  105. — HUMAN  EMBRYO  OF  18.1  MM.    SERIES  1129.     X  5  diams. 


154 


THE  HUMAN  EMBRYO. 


embryo  Ltz,  of  which  he  fixes  the  probable  age  as  just  over  seven  weeks.  The 
head  is  nearer  its  final  position  than  in  figure  103,  and  relatively  larger  in  propor- 
tion to  the  body,  In  the  eye,  cornea  and  conjunctiva  are  clearly  separated;  the 
face  has  the  fetal  form,  the  nose,  mouth,  and  chin  being  fully  marked  off.  The 
arms  are  clearly  divided  into  upper  and  lower  segments;  the  five  digits  are  well 
developed;  the  hands  rest  over  the  heart  and  nearly  touch  one  another.  The  leg 


FIG.  106. — HUMAN  EMBRYO  OF  22.8  MM.     SERIES  871.     X  5  diams. 

shows  the  triparite  division;  the  toes  are  just  beginning  to  be  free,  but  the  hind- 
limb  is  much  less  advanced  than  the  fore-limb.  The  tail  is  still  a  freely  projecting 
appendage. 

Embryos  of  Fifty-three  Days,  22-23  mm- — The  specimen  (Fig.  106)  is  probably 
quite  normal.  As  compared  with  the  last  stage,  there  are  comparatively  few  changes 
of  external  form;  the  most  noteworthy  are  perhaps  the  increased  development  of  the 
legs  and  feet  and  the  commencing  disappearance  of  the  free  tail.  At  this  time  the 


EMBRYO  OF  SIXTY-TWO  DAYS. 


155 


FIG.  107. — HUMAN  EMBRYO  OF  30  MM.     SERIES  913.     X  5  diams. 


156 


THE  HUMAN  EMBRYO. 


protrusion   of    the   coils  of    the   intestine   into   the   ccelom   of    the   umbilical   cord    is 
about  at  its  maximum. 

Embryos  of  Sixty-two  Days,  30  mm. — The  present  specimen  (Fig.  107)  came 
with  no  data  and  its  age  is  therefore  estimated  only.  The  head  is  still  larger  in 
proportion  to  the  body  than  in  figure  106.  The  face  shows  the  two  lines  which,  as 
seen  in  profile,  mark  the  two  ridges  which  run  over  the  cheek,  one  alongside  the 
nose  to  the  corner  of  the  mouth,  the  other  from  the  eye;  these  ridges  are  highly 
characteristic  of  the  ninth  week,  and  traces  of  them  not  rarely  persist  in  the  adult 
physiognomy.  The  limbs  have  grown  considerably,  the  hands  being  lifted  toward 
the  face;  at  the  elbow  there  is  a  considerable  bend;  the  toes  are  all  free,  and  the 
soles  of  the  feet  are  turned  each  toward  the  other.  The  tail  has  disappeared  as  a 
free  appendage.  The  external  genitalia  are  considerably  developed;  the  clitoris-penis 
projects  some  distance. 

Embryo  of  Sixty-four  Days,  32  mm. — A  specimen  came  with  the  following  his- 
tory: "January  4,  1886,  last  flow  began;  March  13,  1886,  abortion";  between  these 
two  dates  are  sixty-eight  days;  but  as  the  flow  took  place,  conception  probably 
occurred  after  menstruation,  therefore  if  we  deduct  four  days,  making  the  age 
sixty-four  days,  we  shall  probably  be  not  far  wrong.  The  head  has  not  yet  as- 
sumed its  final  angle  with  the  body.  On  the  other  hand,  the  protuberance  of  the 

abdomen  is  much  reduced,  so  that  the  body  as  a  whole 
has  begun  to  have  a  more  slender  form  than  in  earlier 
stages.  In  this  specimen  the  eyelids  have  not  even  begun 
to  meet;  in  another  they  have  met,  except  just  in  the 
center,  where  is  still  a  loophole. 

Embryo  of  Seventy-five  Days,  55  mm. — We  figure  next 
(Fig.  108).  a  fetus  concerning  which  there  are  no  data. 
Comparison  with  embryos  of  two  and  three  months  leads 
us  to  place  it  a  little  under  half  way  between  them.  The 
specimen  has  essentially  the  configuration  of  the  young 
child;  but  the  head  is  very  large  and  the  body  slender;  the 
position  of  the  limbs  is  typical;  the  upper  arm  is  bent 
down,  the  forearm  extends  toward  the  chin;  -the  knee  is 
bent  so  as  to  throw  the  foot  toward  the  median  line;  the  soles 
of  the  feet  are  placed  obliquely  facing  one  another;  the  anlages 
of  the  nails  can  be  recognized  on  both  the  fingers  and  toes. 

Embryos  of  the  eleventh  and  twelfth  weeks  are  very  rarely  obtained.  I  have 
never  had  a  normal  one  of  this  period  with  data  to  determine  the  age. 

Embryos  of  Three  Months,  78  to  80  mm.  In  my  experience  there  is  no  other 
age  at  which  abortion  of  normal  embryos  occurs  so  frequently  as  at  three  months, 
and  I  possess  a  number  of  specimens  of  this  age,  which  agree  very .  closely  with 
one  another  in  size  and  form.  The  fetus  drawn  in  figure  109  may  be  taken  to 
represent  accurately  the  appearance  of  the  human  embryo  at  three  months.  The 


FIG.  1 08.— HUMAN  EMBRYO  OF 
55  MM.  SEVENTY-FIVE 
DAYS.  NATURAL  SIZE. 


EMBRYO  OF  FOUR  MONTHS. 


157 


position  of  the  limbs  is  typical  for  this  age,  but  there  are  slight  variations,  in  that 
the  hands,  one  or  both,  may  project  more  and  be  higher  or  lower;  usually  the  right 
foot  lies  in  front  of  the  left,  but  not  always.  Figure  no  gives  the  front  view  of 
the  face  of  the  same  embryo  to  show  the  closed  eyelids,  the  broad  triangular  nose, 
the  thick  lips,  and  the  pointed  chin. 

Embryos  of  Three  and  One-half  Months,  108  to  no  mm. — I  have  several  speci- 
mens which  represent  this  age.  Two 
of  them  are  figured,  one  to  show  the 
natural  attitude  (Fig.  in)  in  utero, 
the  other  (Fig.  112)  to  show  the 
natural  attitude  assumed  by  the  em- 
bryo when  released  from  its  mem- 
branes. The  first  specimen  came  to 
me  with  no  history,  but  as  it  is  cer- 
tainly a  little  larger  than  several  other 
fetuses  of  about  one  hundred  and  six 
days,  it  is  probably  a  little  older. 


FIG.    109.  —  HUMAN   EMBRYO   OF   78   MM.     THREE 
MONTHS.     NATURAL  SIZE. 


.Fie.  no.  —  FRONT  VIEW  or   THE   FACE   OF  THE   EM- 
BRYO  SHOWN  IN  FIGURE  109.     NATURAL  SIZE. 


The  head  is  bent  forward  (Fig.  in);  the  back  is  curved;  the  arms  and  legs  are  both 
raised  toward  the  face;  the  right  leg  is  nearly  straight,  so  that  the  toes  are  brought 
against  the  forehead,  while  the  left  leg  is  bent  at  the  knee,  bringing  the  left  foot 
against  the  right  thigh.  In  this  attitude  the  embryo  fills  out  as  perfectly  as  possi- 
ble an  oval  space,  and  fits,  therefore,  the  cavity  of  the  uterus.  The  second  speci- 
men (Fig.  112)  shows  the  attitude  assumed  by  the  embryo  when  free,  and  proves 
that  the  position  in  utero  (Fig.  in)  is  a  constrained  one.  This  fetus  was  received 
November  30,  1883.  The  delivery  took  place  on  the  morning  of  that  day,  and  the 
last  menstruation  had  ceased  one  hundred  and  six  days  previously;  the  remarkably 
fresh  condition  of  the  fetus  indicated  that  it  had  been  dead  only  a  very  short  time, 
so  that  we  cannot  be  far  wrong  in  putting  its  exact  age  at  one  hundred  and  six  days. 

Embryo  of  Four  Months,  155   mm.—  The   fetus   shown  in   figure  113    came   in  a 


158 


THE  HUMAN  EMBRYO. 


very  fresh  condition,  January  2,  1887;  with  the  statement:  "Conception  said  to 
have  taken  place  September  i,  1886;  fetus  came  away  January  2,  about  noon." 
The  embryo  is  typical  in  size  and  development  for  four  months,  except  that  the 


FIG.   in. — HUMAN  EMBRYO  OF   120  MM.     (?ONE 
HUNDRED  AND  TEN  DAYS.)    NATURAL  SIZE. 


FIG.  112. — HUMAN    EMBRYO    OF    118    MM.     ONE 
HUNDRED  AND  Six  DAYS.     NATURAL  SIZE. 


crown  is   higher  than   usual,   and   the  antero-posterior  diameter  of  the   head  some- 
what below  the  average. 

The  natural  attitude  in  utero  is  similar  to  that  of  figure  in;  the  attitude  shown 
is  that  assumed  by  the  fetus  when  released  from  the  membranes. 


EMBRYO  OF  FOUR  MONTHS. 


159 


FIG.  113. — HUMAN  EMBRYO  OF  155  MM.     ONE  HUNDRED  AND  TWENTY-THREE  DAYS.     NATURAL  SIZE. 


CHAPTER  IV. 

STUDY  OF  THE  SEGMENTATION  OF  THE  OVUM  AND  OF  THE 
BLASTODERMIC  VESICLE  IN  MAMMALS. 

In  selecting  material  for  general  laboratory  work  on  the  early  stages  of  mam- 
mals, we  are  governed  by  practical  considerations.  The  white  mouse  and  the 
rabbit  are  both  easily  kept  in  the  laboratory  and  their  breeding  may  be  accurately 
determined.  Up  to  the  present  time  the  earliest  phases  of  the  development  of  the 
mammalian  embryo  have  been  far  more  thoroughly  studied  in  the  white  mouse  than 
in  any  other  mammal.  For  the  next  following  stages  the  same  remark  applies  to 
the  rabbit.  Hence  these  two  forms  have  been  chosen  for  the  practical  study. 

The  Maturation,  Fertilization,  and  Segmentation  of  the  Ovum  in  White  Mice. 

These  animals  are  selected  for  the  practical  study  of  the  earliest  stages  of 
development  for  two  reasons:  first,  because  the  processes  have  been  more  thoroughly 
studied  in  them  than  in  any  other  mammals;  and,  second,  because  they  are  easily 
kept  and  breed  freely,  so  that  abundant  material  may  be  secured  with  compara- 
tively little  trouble.  Those  desiring  further  information  are  referred  to  Sobotta's 
and  Kirkham's  original  memoirs.* 

Heat  occurs  twenty-one  days  after  littering,  a  fact  which  may  be  taken 
advantage  of  to  secure  ova  of  the  desired  age.  Coitus  can  take  place  only  during 
heat,  for  it  is  then  only  that  the  vagina  is  found  open;  at  other  times  its  epithelium 
concresces  to  a  solid  mass.  The  spermatozoa  do  not  penetrate  into  the  tube  until 
some  time  after  the  coitus.  After  the  discharge  of  the  semen,  the  contents  of 
the  large  seminal  vesicle  are  ejaculated  into  the  vagina,  completely  filling  it  and 
hardening  into  a  white  plug  (bouchon  vaginal),  as  in  guinea-pigs.  From  twenty  to 
thirty  hours  later  the  plug  softens  and  falls  out. 

The  uterine  tubes  are  narrow,  much  contorted  canals.  The  fimbriate  opening 
of  the  tube  penetrates  the  connective  tissue  about  the  ovum  so  that  the  fimbriae 
lie  in  the  periovarial  space.  There  is  ciliated  epithelium  in  the  proximal  region  of 
the  tube  only,  none  in  the  distal  parts  or  in  the  uterus  itself.  During  heat  the 
periovarial  space  is  filled  with  an  abundant  clear  fluid.  This  also  distends  the 


*  Sobotta,  "Die  Befruchtung  und  Furchung  des  Eies  der  Maus,"  Arch.  /.  mikrosk.  Anal.,  vol.  XLV,  15-93, 
PI.  II-IV  (1895). 

Kirkham,  "The  Maturation  of  the  Egg  of  the  White  Mouse."  Trans.  Connecticut  Academy,  xiu,  65-87, 
PI.  I-VIII.  (Corrects  several  important  errors  of  the  preceding  paper.)  Also,  Biol.  Bull.,  (1910)  XVIII,  245. 

160 


POLAR  GLOBULES  IN  WHITE  MICE. 


161 


proximal  part  of  the  tube,  forming,  as  it  were,  a  special  sac,  with  a  distended 
epithelial  lining.  At  the  time  of  coitus  ovulation  has  generally  taken  place;  the 
ovum,  still  surrounded  by  the  cells  of  the  corona  radiata,  is  found  in  the  fluid  of 
the  distended  proximal  section  of  the  tube.  It  is  probable  that  the  ova  are  carried 
from  the  periovarial  space  not  only  by  the  currents  created  by  the  cilia  of  the 
fimbriate  opening,  but  also  by  a  sort  of  pumping  action  of  the  tube  itself.  For 
at  the  beginning  of  the  period  of  heat  we  find  that  the  periovarial  space  contains 
much  fluid,  but  later,  when  the  ova  are  in  the  tube,  this  space  is  empty  and  the 
tube  contains  fluid.  The  ovum  of  the  mouse  measures  only  80^  or  less  in 
diameter,  and  is  therefore  the  smallest  known  mammalian  ovum.  (The  ovum  of 
the  cat  measures  200^,  of  the  rabbit  161^.)  It  is  surrounded  by  a  very  thin 
zona  pellucida  (i6-36//),  and  contains  only  a  few  yolk  grains,  a  portion  of  which 
may  be  blackened  by  osmic  acid.  These  ova  offer  the  further  special  peculiarity 
that  the  first  polar  globule,  which  is  always  formed  in  the  ovary,  is  lost  in  80-90 
per  cent  of  the  ova,  probably  by  extrusion  through  the  zona  pellucida,  so  that 
even  after  the  formation  of  the  second  globule,  they  still  often  have  Only  a  single 
globule  within  the  zona.  The  second  globule  is  produced  only  after  the  ovum  has 
been  transferred  to  the  uterine  tube,  and  then  only  after  a  spermatozoon  has 
entered.  The  process  for  formation  of  the  first  and  second  globules  is  not  the 
same,  although  there  is  a  general  similarity. 

The  First  Polar  Globule. — The  first  polar  globule  is  formed,  as  stated,  while  the 
ovum  is  still  in  the  unruptured  Graafian  follicle  of  the  ovary.  The  nucleus  moves 
toward  one  side  of  the  ovum  and  is  there  transformed  into  a 
mitotic  spindle,  the  axis  of  which  is  more  or  less  nearly  at 
right  angles  to  the  radius  of  the  ovum  (Fig.  114).  The  spindle 
itself  is  large,  pointed  at  the  ends,  with  curving  achromatic 
threads.  The  chromosomes  are  probably  twelve  in  number, 
but  they  vary  in  size  and  shape,  and  even  in  number,  which 
has  been  explained  as  the  result  of  precocious  division  of  some 
of  them.  They  gather  themselves  into  an  equatorial  plate.  FlG- 
They  are  elongated,  pointed  at  the  ends,  with  irregular  sides, 
and  are  very  large.  Minute  centrioles  have  been  observed  at 
the  end  of  the  spindle,  but  there  are  no  astral  rays  extending 
from  the  ends  of  the  spindle  into  the  protoplasm.  The  chro- 
mosomes become  somewhat  V-shaped.  They  divide  by  a  trans- 
verse separation  at  the  apex  of  the  V.  Chromosome  halves  migrate  toward  the  end 
of  the  spindle.  The  stages  occur  probably  about  twenty-four  hours  before  the 
rupture  of  the  follicle.  The  spindle  now  assumes  a  radial  position,  and  one  of  its 
poles  lies  close  to  the  surface  of  the  ovum,  which  has  meanwhile  diminished  in  size 
so  that  there  is  a  considerable  space  between  the  yolk  and  the  zona  pellucida. 
Division  occurs  and  the  first  polar  globule  is  formed,  and  lies  in  the  perivitelline 
space.  In  the  mouse  it  is  remarkable,  as  is  also  the  second,  but  smaller,  polar 


114. — OVUM  OF 
WHITE-  MOUSE,  WITH 
THE  FIRST  POLAR 
SPINDLE  IN  TANGEN- 
TIAL POSITION.  X  500 
diams. —  (After  J. 
Sobotta.) 


162 


STUDY  OF  THE  SEGMENTATION  OF  THE  OVUM. 


globule,  for  its  large  size.  It  is  usually  spherical  in  fresh,  oval  in  preserved 
specimens,  and  measures  in  the  living  state  from  22-28/4  in  diameter.  It  has  a 
distinct  cell-membrane,  a  protoplasm  which  resembles  that  of  the  ovum,  and  may 
even  contain  granules  of  yolk.  Soon  after  its  separation  from  the  ovum  its 
nucleus  becomes  well  developed  and  membranate.  Except,  therefore,  that  the 
number  of  chromosomes  which  enter  into  its  formation  is  half  the  normal  number, 
we  might  say  that  it  differs  little  from  an  ordinary  cell. 

The  Second  Polar  Globule. — After  the  formation  of  the  first  polar  globule 
ovulation  takes  place,  and  during  the  next  changes  the  ovum  is  situated  in  the 
ampulla  of  the  Fallopian  tubes.  In  the  mouse,  unless  the  ovum  is  fertilized,  it 
forms  no  second  polar  globule,  but  instead  undergoes  autolysis  either  in  the  ovary 
or  in  the  uterine  tube.  The  nucleus  of  the  ovum  does  not  enter  into  a  condition 
of  repose,  but  at  once  transforms  itself,  as  in  other  animals,  into  tl)e  second  polar 
spindle.  After  the  constricting  off  of  the  first  polar  globule,  twelve  half  chromo- 
somes (dyads)  are  left  in  the  ovum.  They  are  drawn  into  the  equator  of  a  new 
spindle  and '  split  longitudinally. 


Pg.2 


Spz. 


FIG.  115. — OVUM  OF  WHITE  MOUSE,  DIVIDING  TO 

PRODUCE  THE  POLAR  GLOBULE. 
P.sp.2,     Second     polar     spindle.     Spz,     Head    of 

spermatozoon.        X  500     diams.       (After     J . 

Sobotta.) 


FIG.  116. — OVUM  OF  WHITE  MOUSE,  SHOWING  THE 

METAPHASE  OF  THE  DIVISION  PRODUCING  THE 

FIRST  POLAR  GLOBULE. 
Pg.   2,   Second   polar    globule,     pi,  Cell-plate.     ?, 

Female    pro-nucleus.      X  1500   diams. — (After 

J.  Sobotta.) 


The  second  polar  spindle  is  smaller  than  the  first.  It  lies  at  right  angles  to 
the  axis  of  the  ovum  and  quite  close  to  the  surface.  It  contains  twelve  thick 
achromatic  fibers,  which  do  not  unite  at  the  poles  with  one  another,  but  end  par- 
allel, so  that  the  tip  of  the  spindle  is  blunted.  The  chromosomes,  when  the  mem- 
brane first  disappears,  lie  irregularly,  but  shortly  after  the  formation  of  the  spindle 
they  collect  together  to  form  an  equatorial  plate,  somewhat  as  in  the  figure. 
They  are  irregular  and  of  uneven  size,  twelve  in  number,  or  possibly  the  number 
may  vary  somewhat.  The  chromosomes  then  divide  transversely,  and  the 
halves  move  rapidly  toward  the  ends  of  the  spindle,  which  during  this  change 
passes  into  the  radial  position  (Fig.  115).  The  twenty-four  univalent  chromosomes 
lengthen  into  filaments  of  various  sizes,  and  by  their  form  the  second  spindle  can 


FERTILIZATION  OF  OVUM  IN  WHITE  MICE. 


163 


be  readily  identified.  The  surface  of  the  ovum  or  the  apex  of  the  spindle  forms  a 
protuberance.  Division  of  the  achromatic  fibers  takes  place,  and  there  is  formed  a 
well-marked  cell-plate  (Fig.  116),  and  presently  the  polar  globule  becomes  con- 
stricted off.  The  second  body  is  smaller  than  the  first,  measuring  from  7-1 2^ 
in  diameter,  and  in  the  majority  of  cases  is  the  only  one  to  be  found  inside  the 
zona  after  fertilization.  Its  twelve  chromosomes  soon  form  a  resting  membranate 
nucleus.  The  cell-plate  appears  with  unusual  distinctness.  It  is  at  about  this 
stage  that  the  spermatozoon  is  found  to  have  entered  the  ovum  (Fig.  117,  B)  and  to 
have  formed  there  the  male  pro-nucleus.  During  all  these  stages  no  centrosome 
appears  at  the  poles  of  the  spindle,  but  centrioles  are  said  to  have  been  observed 
at  the  spindle  apices.  No  astral  rays  appear  in  the  protoplasm,  although  in  many 


Pg.2  Pg.l 


FIG.  117. — Two  OVA  OF  WHITE  MOUSE.     A,  WITH  TWO  POLAR  GLOBULES.     B,  WITH  THE  SECOND  POLAR  GLOBULE 

ONLY. 

Pg.  i,  First  polar  globule.     Pg.  2,  Second  polar  globule.    6,  Male  pro-nucleus.     9  Female  pro-nucleus. 

X  500  diams. — (After  J.  Sobotta.) 

eggs  these  astral  figures  are  extremely  conspicuous.  The  female  pro-nuclear  ele- 
ments appear  at  first  as  a  dense  cluster  of  chromatin  granules  (Fig.  117,  J3?),  and 
fuse  apparently  into  a  compact  mass,  which  grows  rapidly  in  size,  presumably  by 
the  absorption  of  fluid  from  the  yolk,  and,  as  it  enlarges,  acquires  a  more  distinct 
outline,  and  presently  shows  a  network  structure  in  its  interior  (Fig.  117,  A),  with 
irregular  chromatin  masses.  It  continues  to  grow  more  and  more,  and  develops 
at  the  same  time  a  series  of  nucleoli  more  or  less  uniform  in  size.  This  stage 
may  be  regarded  as  that  of  the  completed  female  pro-nucleus. 

Fertilization  occurs  in  the  ampulla  of  the  uterine  tube  about  6-10  hours  after 
the  coitus.  Unless  it  occurs  the  development  of  the  second  polar  globule  does  not 
take  place.  It  is  accomplished,  normally,  by  the  penetration  of  a  single  spermato- 
zoon into  the  yolk.  The  tail  of  the  spermatozoon  usually  enters  the  egg  at  least 
in  part.  The  head  of  the  spermatozoon  can  be  recognized  at  first  by  its  shape 
(Fig.  116,  s).  In- position  it  is  typically  more  or  less  remote  from  the  polar  spindle. 
While  the  second  polar  globule  is  forming  the  head  assumes  a  rounded  form,  and 
becomes  the  male  pro-nucleus  (Fig.  117,  s).  The  group  of  twelve  chromosomes  left 


164 


STUDY  OF  THE  SEGMENTATION  OF  THE  OVUM. 


in  the  ovum  after  the  division  of  the  polar  spindle  becomes  the  female  pro-nucleus 
(Fig.  117,  9).  Both  pro-nuclei  now  enlarge,  the  female  most,  and  assume  a  nearly 
spherical  form,  but  have  no  membrane  (Fig.  117,  A).  They  approach  one  another, 
drawing  also  toward  the  center  of  the  ovum,  until  they  come  to  lie  side  by  side, 


FIG.  118. — OVUM  OF  WHITE 
MOUSE.  BEGINNING  OF  THE 
CONJUGATION  OF  THE  PRO- 
NUCLEI.  X  isoodiams. — (After 
Sobotta.) 


FIG.  119. — OVUM  OF  WHITE 
MOUSE.  CONJUGATION  OF  THE 
PRO-NUCLEI,  AND  FORMATION  OF 
THE  SEGMENTATION  SPINDLE. 
X  1500  diams. — (After  Sobotta.) 


yet  separated  by  a  small  space.  The  chromatin  of  the'  two  pro-nuclei  forms  dis- 
tinct threads.  Next  there  appears  in  the  space  between  them  a  centrosome  with  a 
few  radiating  lines  around  it  (Fig.  118).  From  the  centrosome  arises,  just  how  is 
not  clear,  a  spindle  of  achromatic  threads  (Fig.  119).  The  chromatin  of  each  pro- 


FIG.  1 20. — OVUM  OF  WHITE 
MOUSE.  FIRST  SEGMENTATION 
SPINDLE  WITH  THE  CHROMO- 
SOMES OF  THE  PRO-NUCLEI  STILL 
FORMING  Two  DISTINCT 
GROUPS.  X  1500  diams. — 
(After  Sobotta.) 


FIG.  121. — OVUM  OF  WHITE  MOUSE.  FIRST  SEG- 
MENTATION SPINDLE  WITH  EQUATORIAL  PLATE 
OF  CHROMOSOMES.  X  1500  diams. — (After 
Sobotta.) 


nucleus  now  forms  a  group  of  well-defined,  elongated,  somewhat  crooked  chromo- 
somes. The  two  groups  of  chromosomes  are  quite  distinct,  and  are  separated  from 
one  another  by  the  intervening  spindle  (Fig.  119).  The  spindle  continues  to  grow, 
and  the  chromosomes  of  the  male  pro-nucleus  on  the  one  side  and  the  female  pro- 


FERTILIZATION  OF  OVUM  IN  WHITE  MICE. 


165 


nucleus  on  the  other  attach  themselves  to  the  equatorial  region  of  the  spindle  (Fig. 
120).  The  spindle  continues  to  grow;  the  chromosomes  become  V-shaped  and  ar- 
range themselves  as  the  so-called  equatorial  plate,  in  which  the  chromosomes  of 
the  two  pro-nuclei  can  no  longer  be  distinguished  from  one  another  (Fig.  120).  At 
each  end  of  the  spindle  is  a  distinct  centrosome  with  a  very  faint,  small  astral 
radiation  in  the  neighboring  protoplasm.  This  spindle  is  the  beginning  of  the  divi- 
sion of  what  we  may  call  the  segmentation  ,,..,,-..... ^i^^^^^^^.^i,^.,. . 
nucleus.  In  the  mouse  the  two  pro-nuclei  do 
not  actually  fuse  into  a  single  nucleus  before  the 
formation  of  the  spindle,  which  initiates  the  first 
division  of  the  fertilized  ovum,  so  that,  strictly 
speaking,  there  is  no  fusion  of  the  pro-nuclei  to 
make  a  segmentation  nucleus.  There  is,  never- 
theless, a  true  fusion  of  the  pro-nuclei  accom- 
plished, although  it  is  somewhat  masked  by  the 
early  commencement  of  the  first  segmentation 
spindle,  which  develops  at  the  same  time  that  the 
fusion  of  the  pro-nuclei  is  being  completed. 

The  chromosomes  of  the  equatorial  plate  now  divide,  probably  by  splitting  longi- 
tudinally, so  that  the  number  of  chromosomes  is  doubled.  During  the  splitting  the 
chromosomes  tend  to  draw  apart  from  one  another.  At  the  same  time  the  spindle, 
without  changing  its  length,  becomes  somewhat  narrower.  The  chromosomes  now 
move  apart  from  the  equator  toward  the  two  poles,  forming  two  groups,  each  group 


FIG.  122. — OVUM  OF  WHITE  MOUSE.  FIRST 
SEGMENTATION  SPINDLE. 

The  chromosomes  have  divided  and  have 
migrated  toward  the  poles  of  the  spin- 
dle, forming  two  groups.  X  1500 
diams. — (After  Sobotta.) 


p.g.          Z. 


FIG.  123. — OVA  OF  WHITE  MOUSE  WITH  Two  SEGMENTATION  SPHERES  OR  CELLS. 

.4,  Telophase  of  the  division;  the  chromosomes  are  reconstituting  the  nucleus.  B,  Membranate  nucleus  recon- 
stituted. I,  First  cell  of  segmentation,  nu,  Nucleus,  p.g,  Polar  globules.  Z,  Zona  pellucida.  X  500 
diams. — (After  Sobotta.) 


containing  half  of  the  total  number  of  chromosomes  (Fig.  122),  and  at  the  same 
time  the  whole  ovum  becomes  somewhat  elongated  in  the  direction  corresponding 
with  the  axis  of  the  spindle.  The  chromatin  granules  accumulate  at  the  two  poles 
of  the  spindle.  The  achromatic  threads  between  the  poles  break  through.  Then 
the  actual  cleavage  of  the  elongated  ovum  into  two  cells  becomes  marked  in  the 


166  STUDY  OF  THE  SEGMENTATION  OF  THE  OVUM. 

protoplasm,  and  the  line  of  separation  of  the  two  cells  passes  through  the  equator 
of  the  spindle  (Fig.  123,  A).  The  accumulated  granules  of  chromatin  then  reconsti- 
tute the  resting  membranate  nucleus  (Fig.  123,  B).  In  brief,  the  segmentation  of 
the  ovum  is  a  typical  indirect  or  mitotic  cell-division.  In  the  mouse  the  first 
cleavage  is  completed  about  twenty-six  hours  after  the  coitus.  The  second  cleav- 
age is  not  completed  until  twenty-four  hours  later.  When  first  formed,  the  two 
segmentation  spheres  are  oval  and  entirely  separated  from  one  another,  but  subse- 
quently they  flatten  against  one  another  and  become  appressed,  a  phenomenon  of 
which  we  have  no  explanation.  In  most  mammals  which  have  been  studied  there 
is  more  or  less  space  between  the  segmenting  ovum  and  the  zona  (see  Fig.  6),  but 
in  the  mouse  this  space  is  reduced  to  a  minimum  and  the  zona  is  often  stretched 
into  irregular  forms  during  the  changes  of  the  ovum. 

Method  of  Obtaining  Blastodermic  Vesicles  from  the  Rabbit. 

The  does  should  be  allowed  to  become  pregnant  and  be  isolated  until  they 
have  littered;  the  date  of  littering  should  be  noted,  and  thirty  days  thereafter  the 
buck  be  admitted  and  the  exact  time  of  the  covering  recorded.  At  the  proper 
number  of  days  thereafter  the  animal  should  be  killed  and  the  uterus  removed  at 
once.  It  may  be  opened  with  two  pairs  of  forceps  used  to  split  the  outer  muscular 
walls  of  the  organ,  beginning  the  operation  at  the  lower  end  of  the  uterus.  With  a 
little  care  this  can  be  done  without  rupturing  the  mucous  membrane,  which  is  to 
be  afterward  also  opened  in  a  similar  manner  with  the  forceps  and  the  blastoder- 
mic  vesicles  exposed.  They  are  small  bodies  of  rounded  form  and  with  a  brilliant 
pearly  luster,  and  are  easily  observed.  During  the  earlier  stages,  which  occur  in 
the  Fallopian  tubes,  the  ova  are  very  small  and  difficult  to  find,  but  by  the  time' 
the  ovum  has  reached  the  uterus  it  has  become  a  blastodermic  vesicle  measuring 
about  0.6  mm.  in  diameter,  and,  therefore,  easily  seen  by  the  naked  eye.  From 
the  fourth  day  after  coitus  until  the  beginning  of  the  seventh  day  the  vesicles  lie 
free  in  the  uterus.  Usually  early  in  the  seventh  day  the  vesicles,  which  then 
measure  about  4.5  by  3.5  mm.,  begin  to  attach  themselves  to  the  wall  of  the 
uterus,  and  thereafter  are  much  more  difficult  to  remove.  At  the  beginning  of  the 
fifth  day  the  ova  measure  about  0.6  to  0.9  mm.  in  diameter,  but  vary  greatly  in 
size,  and  are  found  more  or  less  near  together  in  the  upper  portion  of  the  oviduct. 
By  the  end  of  the  sixth  day  they  measure  about  4.0  mm.  and  are  distributed 
throughout  the  entire  length  of  the  uterus. 

The  most  useful  stages  are  the  vesicles  from  the  beginning  of  the  sixth  and 
seventh  days.  To  preserve  the  vesicles  they  must  be  gently  removed  from  the 
uterus,  great  care  being  necessary  not  to  injure  them,  and  dropped  into  Zenker's 
or  Hermann's  fluid.  In  either  of  these  they  may  be  left  for  about  an  hour  and 
then  washed  and  preserved  in  the  usual  manner.  Specimens  should  be  examined 
in  the  fresh  state,  just  after  they  have  been  preserved,  and  after  they  have  been 
stained,  before  they  are  imbedded.  For  staining,  alum  cochineal  or  borax  carmine 


STUDY  OF  RABBIT  BLASTODERMIC  VESICLES  IN  ALCOHOL.          167 

is  recommended.  Finally,  the  specimens  are  to  be  imbedded  in  paraffin  and  cut 
in  series  in  the  usual  manner;  sections  of  from  6  to  8//  are  desirable.  Unfor- 
tunately, no  method  has  yet  been  devised  by  which  these  delicate  vesicles  may  be 
imbedded  without  distortion  of  their  form,  so  that,  when  the  sections  are  finally 
obtained,  the  blastodermic  walls  are  wrinkled  and  more  or  less  out  of  shape.  But 
fortunately,  owing  apparently  to  its  greater  thickness,  the  embryonic  area  usually 
escapes  distortion  and  appears  in  the  sections  of  normal  form,  or  nearly  so. 

Study  of  Rabbit  Blastodermic  Vesicles  in  Alcohol. 

All  of  the  most  important  points  in  the  structure  of  the  blastodermic  vesicles 
of  the  rabbit  from  the  fourth  to  the  seventh  day  may  be  fairly  well  observed  by 
examining  the  hardened  vesicles  in  alcohol  under  the  microscope.  For  such  exami- 
nations the  so-called  live-box,  such  as  was  formerly  much  used  by  microscopists 
for  the  study  of  living  creatures,  will  be  found  very  convenient.  Care  must  be 
taken  to  have  plenty  of  alcohol  around  the  specimen  and  not  to  lower  the  cover 
so  much  as  to  exert  any  pressure  upon  the  vesicle.  It  is  not  difficult  to  place  the 
vesicles  so  that  any  part  of  their  surface  may  be  examined  with  a  No.  7  objective. 
In  the  uncolored  specimen  the  nuclei  and  even  many  of  the  boundaries  of  the 
cells  can  be  clearly  made  out. 

In  the  following  descriptions  ages  have  been  chosen  at  which  the  important 
characteristics  can  usually  be  observed.  The  variation  is  so  great  in  range  during 
early  stages  that  the  development  described  below  for  a  given  age  is  often  found 
in  older  or  younger  specimens,  and  specimens  of  a  given  age  may  exhibit  a  less 
or  a  more  advanced  stage  of  the  embryonic  formation  than  is  here  put  down 
for  that  age.  In  general  the  correspondence  of  the  stage  of  development  to  the 
size  of  the  vesicle  is  more  exact  than  to  its  age. 

Vesicles  at  Five  Days  (5  X  24  hours). — At  this  age  the  vesicles  are  always  found 
in  the  upper  portion  of  the  uterus.  Sometimes  all  of  those  in  one  uterus  are  quite 
close  together,  at  other  times  somewhat  scattered  and  lying  singly.  The  vesicles 
are  extremely  variable  in  size,  for  they  measure  from  0.6  to  0.9  mm.  They  are 
spherical  or  nearly  so,  and  are  surrounded  by  a  thin  membrane,  which  in  reality 
corresponds  to  both  the  zona  pellucida  and  the  outer  albuminous  envelope,  which 
in  the  rabbit  ovum  during  segmentation  is  very  thick  and  conspicuous,  but  which 
is  always  extremely  thin  when  the  stage  of  the  blastodermic  vesicle  is  reached. 
Upon  the  outside  of  this  really  double  membrane  appear  a  certain  number  of 
small  villus-like  projections,  which  are  highly  refringent.  They  are  probably  identi- 
cal in  character  with  the  villi  which  have  been  observed  upon  the  ovum  of 
the  dog  (page  45),  but  are  smaller  in  all  of  their  dimensions.  Immediately  un- 
derneath the  external  membrane  there  is  a  continuous  layer  of  cells  belonging 
to  the  ectoderm  and  extending  completely  around  the  ovum.  The  layer  is  some- 
times designated  specifically  as  the  " outer  layer"  or  as  the  " subzonal  layer"  It 
also  extends  over  the  embryonic  shield;  the  portion  upon  the  shield  is  often  termed 


168  STUDY  OF  THE  SEGMENTATION  OF  THE  OVUM. 

Rauber's  layer,  it  having  been  first  observed  by  that  investigator.  The  cells  of  the 
outer  layer  are  quite  large  and  their  boundaries  are  easily  recognizable  in  surface 
views.  Their  sides  may  number  four,  five,  or  six,  six  being  perhaps  the  more 
usual  number,  and  are  variously  disposed,  so  that  the  cells  differ  in  shape  and  size. 
During  the  next  two  days  of  development  the  cells  become,  if  anything,  more  ir- 
regular in  outline  and  somewhat  smaller.  The  boundaries  between  the  cells  are 
very  fine  lines;  the  nuclei  are  rather  large  and  oval  in  form,  and  contain  from  three 
to  four  or  five  highly  refringent  granules.  Each  nucleus  is  surrounded  by  a  denser 
court  of  protoplasm  in  which  there  are  many  granules,  some  of  which  are  highly 
refringent.  The  peripheral  portion  of  the  cell  is  of  a  loosely  reticulate  structure 
with  comparatively  wide  meshes  between  the  threads  of  the  protoplasm.  Occasion- 
ally there  appear  in  the  protoplasm  of  these  cells  narrow,  elongated,  highly  refrin- 
gent bodies  somewhat  resembling  bacilli  in  appearance,  and  therefore  they  are 
termed  the  bacilliform  bodies.  Their  nature  is  unknown;  they  are  more  apt  to  be 
found  in  older  vesicles.  The  outer  or  subzonal  layer  can  be  made  out  over  the 
embryonic  shield  only  by  very  careful  observation.  In  the  shield  the  cells  are  sev- 
eral layers  thick.  The  inner  cells  are  very  much  smaller  in  size  than  the  cells  of 
the  outer  layer,  are  more  granular,  and  contain  smaller  nuclei  which  take  up  a 
relatively  large  place  in  the  cell  in  proportion  to  its  apparent  area.  Closer  observa- 
tion, utilizing  the  fine  adjustment  of  the  microscope,  will  show  that  there  are  two 
kinds  of  cells  in  the  inner  part:  first,  those  which,  like  the  cells  of  the  subzonal 
layer,  belong  to  the  ectoderm;  and,  second,  an  inner  layer  of  cells,  which  appar- 
ently belongs  entirely  to  the  entoderm.  In  the  region  of  the  embryonic  shield  the 
ectoderm  is,  therefore,  made  up  of  two  distinct  layers  of  cells.  The  outer  or  sub- 
zonal  (Rauber's  layer)  disappears  during  the  sixth  day  of  development  as  a  distinct 
layer.  The  cells  of  the  entoderm  form  a  very  thin  continuous  layer  on  the  under 
side  of  the  embryonic  shield.  They  may  be  recognized  by  the  very  granular,  and 
therefore  dark,*  appearance  of  their  protoplasm,  and  by  the  rounded  form  and 
small  size  of  their  nuclei.  Similar  cells  may  be  observed  also  extending  beyond 
the  limits  of  the  embryonic  shield,  though  not  there  forming  a  continuous  layer, 
except  perhaps  for  a  very  short  distance,  but  rather  lying  scattered  about  in  patches 
or  isolated.  As  the  cuboidal  cells  of  the  ectoderm  are  confined  to  the  region  of 
the  embryonic  shield,  the  cells  of  the  entoderm  outside  of  the  shield  lie  close 
against  the  subzonal  layer.  Here  they  may  be  more  easily  studied  than  in  the 
shield  itself.  They  are  very  much  smaller  than  the  cells  of  the  outer  layer  and 
contain  each  a  nucleus  with  highly  refringent  granules,  which  are  now  numerous 
and  smaller  than  the  somewhat  similar  granules  in  the  overlying  nuclei  of  the  ecto- 
derm. The  farther  away  we  proceed  from  the  edge  of  the  embryonic  shield,  the 
fewer  we  find  the  entodermal  cells.  The  extent  of  their  distribution  varies  greatly, 
and  apparently  more  or  less  in  relation  to  the  size  of  the  blastodermic  vesicle,  since 


*  As  seen  by  transmitted  light 


STUDY  OF  RABBIT  BLASTODERMIC  VESICLES  IN  ALCOHOL. 


169 


in  the  smallest  vesicles  of  this  age  we  find  the  cells  only  a  short  distance  beyond 
the  edge  of  the  shield,  yet  in  the  largest  vesicles  they  have  expanded  even  past  the 
equator. 

Vesicles  at  Six  Days. — At  this  age  the  vesicles  are  found  more  or  less  scat- 
tered and  isolated  in  position  from  one  another  through  the  upper  half  of  the 
uterus.  They  are  nearly  spherical  and  measure  from  i .  o  to  1.6  mm. ;  their  walls 
are  very  transparent  and  the  somewhat  more  opaque,  round  or  oval  embryonic 
shield  can  be  readily  distinguished  with  a  hand  lens  (Fig.  124).'  Its  size  varies 
with  the  diameter  of  the  vesicle,  being  larger  in  the  larger  vesicles;  but  the  pro- 
portions are  not  exact,  for  a  vesicle  of  given  diameter  may  have  an  embryonic 
shield  of  either  larger  or  smaller  dimensions  than  other  vesicles  of  the  same  size. 
Hence,  vesicles  of  different  sizes  may  have  embryonic 
shields  of  similar  dimensions.  The  actual  diameter  of 
the  shield  is  between  0.2  and  0.35  mm.  The  general 
structure  of  the  vesicles  is  the  same  as  at  five  days, 
but  certain  differences  may  be  noted.  In  preserved 
specimens  the  external  membrane  is  very  apt  to  be 
wrinkled.  The  subzonal  layer  has  very  much  the  same 
appearance  as  before,  though  the  cells  are  somewhat 
smaller  and  it  has  almost  disappeared  over  the  region 
of  the  embryonic  shield.  The  manner  of  its  disap- 
pearance has  not  been  definitely  settled.  There  is  no  FIG.  124. — BLASTODERMIC  VESI- 
evidence  that  the  cells  degenerate  or  are  cast  off,  hence 
one  inclines  to  the  hypothesis  that  the  cells  of  the 
subzonal  layer  become  incorporated  in  the  inner  layer 
of  the  cuboidal  ectodermal  cells,  for  in  sections  shown  at 

this  stage  the  ectoderm  is  one-layered  in  the  region  of  the  shield.  The  entodermal  cells 
also  have  essentially  the  same  appearance  as  at  five  days,  but  they  extend  considerably 
farther  around  the  vesicle,  are  more  numerous,  and  form  a  more  continuous  layer. 
Sections  show  that  the  subzonal  layer  outside  of  the  shield  is  very  thin,  but  its 
outer  surface  is  fitted  to  the  inner  surface  of  the  zona  pellucida.  The  center  of 
each  cell  is  somewhat  thicker,  projecting  toward  the  interior  of  the  vesicle.  It  is 
in  this  thicker  projecting  portion  that  the  nucleus  is  situated.  Along  the  borders  of 
the  cells  the  layer  is  of  course  thinner,  and  it  is  under  these  thinner  parts  that  the 
thicker  nucleated  portions  of  the  entodermal  cells  are  lodged.  Hence,  in  surface 
views,  the  nuclei  of  the  two  layers  are  seen  to  alternate  more  or  less  with  one  an- 
other. This  characteristic  disposition  is  not  kept^  everywhere,  but  is  subject  to 
considerable  variations.  In  the  very  most  advanced  ova  of  six  days  a  small  spot 
sometimes  can  be  observed  in  the  embryonic  shield  which  is  noticeable  from  its 
greater  opacity.  This  spot  corresponds  to  Hensen's  knot,  but  it  does  not  usually 
show  itself  distinctly  until  considerably  later. 

Vesicles  at  Seven  Days. — Vesicles  at  this  age  vary  greatly  in  size,  and  the  stage 


CLE  OF  A  RABBIT  OF  Six  DAYS 
AND  ONE  AND  ONE-HALF  HOURS. 
FROM  AN  ALCOHOLIC  SPECIMEN. 
X  20  diams. 


170  STUDY  OF  THE  SEGMENTATION  OF  THE  OVUM. 

of  development  varies  with  the  size— how  exactly,  we  do  not  yet  know.  Prelimi- 
narily we  may  fix  on  the  normal  size  as  being  that  of  vesicles  the  greatest  diam- 
eter of  which  is  about  4  mm.  Such  vesicles  are  somewhat  oval  in  shape  and  slightly 
flattened  on  the  side  bearing  the  embryonic  shield.  The  membrane  enclosing 
them  is  very  thin;  the  albuminoid  layer  can  scarcely  be  distinguished,  but  the  zona 
pellucida  is  very  distinct.  The  shield  (Fig.  125,  Sh)  is.  somewhat  elongated  and 
distinctly  pear-shaped.  Its  long  axis  is  parallel  with  that  of  the  vesicle.  It  varies 
greatly  in  its  dimensions.  Shields  i  mm.  wide  and  from  1.3  to  1.4  mm.  long  are 
not  uncommon.  The  student  will  be  likely  to  encounter  other  dimensions.  The 
most  striking  addition  is  the  appearance  of  a  darker  area,  mes,  at  the  posterior  or 
pointed  end  of  the  shield.  This  darker  area  is  also  somewhat  pear-shaped,  but 
its  pointed  end  is  near  the  center  of  the  shield,  its  rounded 
end  a  little  distance  behind  the  point  of  the  shield.  The 
darker  area  owes  its  formation  to  the  appearance  of  a  new 
layer  of  cells  between  the  ectoderm  and  entoderm.  This 
layer  consists  of  loosely  connected  cells  with  rounded  nuclei 
easily  distinguishable  in  surface  views  from  those  of  the 
subzonal  layer.  The  greater  part  of  these  cells  are  certainly 
mesodermic,  but  a  portion  of  them  share  in  the  formation 
FIG  i2<c—  BLASTODERMIC  °^  *ne  primitive  streak  and  notochordal  canal,  and  perhaps 
VESICLE  OF  A  RABBIT  do  not  belong  to  the  mesoderm.  In  the  region  outside  the 
AT  SEVEN  DAYS.  embryonic  shield  the  outer  layer  is  easily  distinguished;  its 

Sh,  Embryonic  shield,   mes,  111  i     j  ^T  r  j- 

/c     .  ,.        cells    have    marked    outlines,   but    are    of    smaller    dimensions 

Mesoderm.        (Semi-dia- 

grammatic.)  x  6  diams.  than  in  earlier  stages,  their  nuclei  are  large,  for  the  most 
part  oval,  and  contain  several  highly  refringent  and  con- 
spicuous granules.  The  number  of  granules  varies;  when  there  are  only  two  or 
three,  they  are  apt  to  be  elongated  as  if  several  small  granules  had  united.  The 
entodermal  cells  have  spread  well  past  the  equator  of  the  vesicle  and  present, 
for  the  most  part,  a  distinctly  epithelial  arrangement,  although  at  the  edge  of  the 
expanding  layer  the  cells  are  still  more  or  less  scattered.  The  entodermal  cells 
are  easily  distinguished  by  changing  the  focus  of  the  microscope,  when  their  darker 
protoplasm  and  smaller  size,  together  with  their  smaller  darker  nuclei,  make  them 
readily  recognizable.  The  granules  in  the  entodermic  nuclei  are  smaller  and  more 
numerous  than  in  the  overlying  ectodermal  nuclei. 

During  the  next  few  hours  further  changes  ensue.  At  the  apex  of  the  pear- 
shaped  mesodermic  area  there  appears  a  small  spot,  which  is  known  as  Hensen's 
knot.  At  first  Hensen's  knot  consists  of  a  little  thickening  accompanied  by  a  union 
of  the  cells  of  the  middle  layer  with  those  of  the  overlying  ectoderm.  Next  occurs 
the  development  of  the  primitive  streak,  which  runs  from  Hensen's  knot  backward 
toward  the  apex  of  the  embryonic  shield,  and  very  soon  thereafter  along  the  line 
of  the  primitive  streak  there  develops  the  external  and  shallow  primitive  groove. 
At  Hensen's  knot  the  three  layers  now  are  found  to  bet  intimately  united,  so  that, 


STUDY  OF  RABBIT  BLASTODERMIC  VESICLES  IN  ALCOHOL.          171 

though  they  may  everywhere  else,  when  fresh,  be  separated  from  one  another,  the 
germ-layers  at  this  point  cannot  be  separated,  except  by  tearing.  Finally,  in  the 
next  stage  there  grows  out  in  front  of  Hensen's  knot  the  so-called  head-process,  an 
axial  band  of  cells  in  which  the  notochordal  canal  develops. 


A  ..-vvv^^^;vip^ 

^•>..-i....-^fe''v'  :•.!'-;'<.•;.'•'*«•    '-<t\ 


B 


^^^ 

.<?' 

v'^vr.;^-^KV:!g;i;:' 


'"•" '"£'•&, 

FIG.  126. — THREE  TRANSVERSE  SECTIONS  OF  A  RABBIT  EMBRYO  OF  SEVEN  AND  ONE-HALF  DAYS.     SERIES  622, 

SECTIONS  247,  260,  381.      X  300  diams. 


Examination  of  Cross-sections. — The  structure  of  the  embryonic  shield  is  well 
shown  by  cross-sections.  Figure  126  represents  three  sections  through  the  embryonic 
shield  at  seven  and  one-half  days:  A,  through  the  head  process;  B,  through  Hen- 


. 
172  STUDY  OF  THE  SEGMENTATION  OF  THE  OVUM. 

sen's  knot;  C,  through  the  primitive  groove.  The  three  primitive  germ-layers  are 
easily  recognized  in  each  section.  They  are  somewhat  separated  from  one  another, 
but  in  life  probably  lay  close  together.  The  upper  layer  or  ectoderm  is  the  thick- 
est and  consists  of  low  columnar  cells;  it  is  characteristic  of  the  embryonic  shield, 
and  at  the  edge  (not  included  in  the  figure)  of  the  shield  changes  to  a  thin  sheet 
of  cells,  which  forms  the  outer  layer  of  the  rest  of  the  blastodermic  vesicle.  The 
lowest  layer,  entoderm,  is  a  thin  sheet  only  one  cell  thick.  The  middle  layer, 
mesoderm,  is  more  irregular,  and  has  begun  to  thicken,  being  in  places  two  or 
even  three  cells  thick.  In  the  median  line  the  mesoderm  enters  into  special  rela- 
tions with  the  other  layers.  In  the  middle  section,  B,  which  passes  through  Hen- 
sen's  knot,  it  forms  a  considerable  axial  thickening,  which  fuses  with  the  entoderm 
below  and  ectoderm  above,  and  builds  with  the  latter  a  dome-like  projection. 
The  axial  thickening  extends  forward  from  Hensen's  knot,  constituting  the  so-called 
head-process,  and  in  this  region,  A,  the  mesoderm  is  united  only  with  the  entoderm. 


-  Ent. 


FIG.  127. — RABBIT  EMBRYO  OF  SEVEN  DAYS.     TRANSVERSE  SERIES  12,  SECTION  216,  THROUGH  THE  ANTERIOR 

PORTION  OF  THE  EMBRYONIC  SHIELD. 
EC,  Ectoderm.     Ent,  Entoderm.      X  350  diams. 


Behind  Hensen's  knot  the  thickening  extends  backward,  making  the  primitive 
streak,  C,  which  is  characterized  by  the  union  of  the  mesoderm  with  the  ectoderm 
only,  and  by  the  primitive  groove,  a  shallow  median  longitudinal  depression  of  the 
ectoderm. 

The  structure  of  the  embryonic  shield  at  seven  days  is  further  illustrated  by 
figures  127  and  128,  the  former  passing  across  the  anterior  portion  of  the  shield, 
where  it  is  two-layered,  and  the  latter  across  the  posterior  portion,  in  which  the 
middle  layer  has  appeared.  Figure  176  shows  the  middle  portion  of  a  section.  It 
consists  merely  of  the  outer,  thicker,  ectodermal  layer,  EC,  and  the  very  thin  ento- 
dermal  layer,  Ent.  Both  surfaces  of  the  ectoderm  are  quite  sharply  defined.  The 
nuclei  are  rather  large  and  show  several  large,  deeply  stained  nucleoli  in  each. 
The  outline  of  the  nucleus  is  sharp,  and,  in  addition  to  the  larger  granules,  there 
are  many  smaller  ones  less  deeply  stained  scattered  through  the  nucleus.  The 
nuclei  vary  considerably  in  size,  shape,  and  position.  The  protoplasm  of  the  ecto- 
dermal cells  is  lightly  stained,  and  granular  in  appearance.  The  boundaries  between 
adjacent  cells  are  indicated  by  delicate  lines,  which  extend  through  the  entire  thick- 
ness of  the  ectoderm,  which  is  now  but  a  single  layer  of  cells.  The  original  outer 


STUDY  OF  RABBIT  BLASTODERM  1C  VESICLES  IN  ALCOHOL. 


173 


(Rauber's)  layer  has  disappeared.  The 
entoderm  is  very  thin,  but  is  thickened  a 
little  where  each  nucleus  is  lodged.  The 
nuclei  are  smaller  than  those  of  the  ecto- 
derm, more  darkly  stained,  and  the  gran- 
ules in  them  less  coarse  than  those  in  the 
nuclei  of  the  ectoderm.  Between  the  two 
layers  is  a  narrow  space;  whether  an 
artefact  or  not  is  difficult  to  say.  Figure 
128  represents  a  transverse  section  through 
the  posterior  part  of  the  embryonic 
shield  where  the  primitive  streak,  pr.s,  is 
just  forming.  The  position  of  the  median 
plane  is  approximately  indicated  by  the 
vertical  line  M.  About  this  plane  there 
is  a  considerable  accumulation  of  cells 
which  merges  without  boundary  into  the 
superficial  cells  of  the  shield.  A  short 
distance  from  the  median  line  the  outer 
layer  of  the  shield  becomes  a  distinct  epi- 
thelium, EC,  consisting  of  a  single  layer  of 
cells.  The  edge  of  the  shield  is  marked 
by  a  rather  abrupt  transition  to  the  thin 
outer  layer  of  the  extra-embryonic  region. 
On  the  under  side  of  the  section  extends 
the  thin  entoderm  as  a  continuous  layer, 
which  is  only  loosely  connected  with  the 
central  mass  of  cells  overlying  it  near  the 
median  plane.  Finally,  from  the  median 
mass  of  cells  extends  laterally  the  sheet 
of  mesoderm,  Mes,  between  the  outer 
and  inner  germ-layers.  The  mesodermic 
cells  are  somewhat  loosely  distributed,  and 
have  round  nuclei  with  distinct  chroma- 
tin  granules  and  well-marked  protoplasmic 
bodies,  which  give  off  strands  by  which 
the  cells  are  united  to  one  another.  The 
middle  germ-layer  is  the  least  compact  of 
the  three. 


CHAPTER  V. 
STUDY  OF  YOUNG  CHICK  EMBRYOS. 

Method  of  Obtaining  Embryos. 

Fertile  eggs  can  usually  be  obtained  from  dealers,  who  can  supply  them  in 
quantities  as  needed,  or  hens  may  be  kept  wifh  little  trouble  especially  for  the 
purpose.  In  that  case  the  hen  herself  will  be  found  the  best  incubator,  for  the 
number  of  eggs  which  develop  normally  under  a  hen  is  larger  than  in  an  artificial 
incubator,  and  abnormalities  of  development  are  less  frequent.  A  good  setter  will 
remain  upon  the  eggs,  even  though  some  are  removed  and  replaced  by  fresh  ones, 
for  about  a  month.  She  should  be  plentifully  supplied  with  water  and  soft  food, 
which  is  best  kept  at  a  little  distance  off,  so  that  she  will  be  obliged  to  leave  the 
eggs  to  feed.  A  box  that  is  somewhat  secluded,  and  affords  some  protection, 
warmth,  and  shelter  from  the  light,  should  be  provided.  In  order  to  obtain  the 
most  accurate  results  it  is  desirable  to  place  the  eggs  as  soon  as  laid  immediately 
under  the  hen.  Only  by  this  means  can  an  approximate  correlation  between  the 
stage  of  development  and  the  duration  of  incubation  be  secured. 

Artificial  incubators  are  now  made  to  work  satisfactorily.*  The  temperature 
of  an  incubator  should  be  maintained  at  about  38°  C.  (100.4°  F.).  It  should  on 
no  account  be  allowed  to  rise  above  40°  C.  (104°  F.),  for  that  destroys  a  portion 
of  the  eggs  and  causes  the  production  of  many  abnormalities  in  the  remainder;  and, 
if  possible,  a  fall  to  a  lower  temperature  should  be  avoided,  although  the  results  of 
a  lower  temperature  are  less  disastrous.  No  incubator  should  be  used  which  does 
not  permit  a  constant  supply  of  fresh  air  and  of  moisture.  The  date  should  always 
be  marked  on  each  egg  when  it  is  placed  in  the  incubator.  If  a  number  of  eggs 
from  a  dealer  are  artificially  incubated  the  same  length  of  time,  they  are  pretty  sure 
to  cover  a  considerable  range  of  stages,  as,  of  course,  'eggs  so  supplied  are  of  vary- 
ing ages,  the  exact  time  of  laying  not  being  recorded. 

In  this  work  two  stages  of  the  chick  are  especially  studied.  The  first  stage 
studied  is  that  of  a  chick  with  seven  segments,  which  is  normal  after  about  twenty- 
seven  hours'  incubation.  The  second  is  normally  produced  after  about  forty-six 
hours'  incubation.  The  embryo  should  have  about  twenty-eight  segments  and  three 


*  The  one  used  at  the  Harvard  Medical  School  is  heated  by  a  kerosene  lamp  and  has  a  capacity  of  100 
eggs.  It  is  called  the  New  Method  Incubator,  and  was  purchased  from  M.  A.  Coffin,  Burlington,  Mass.  In  the 
market  other  incubators  may  be  found,  doubtless  equally  good,  among  them  patterns  adapted  for  the  use  of  gas 
where  that  is  preferred. 

174 


METHOD  OF  OBTAINING  EMBRYOS.  175 

gill-clefts  showing  externally.  Embryos  a  little  less  or  a  little  more  developed  are 
almost  equally  serviceable. 

Removing  the  Embryo. — Before  the  egg  is  opened  a  basin  should  be  prepared 
and  filled  with  normal  salt  solution  warmed  to  about  40°  C.  (104°  F.).  The  basin 
should  be  large  enough  to  permit  the  entire  egg  to  be  submerged  in  it. 

Take  the  egg  warm  from  the  incubator  or  the  hen;  allow  it  to  rest  quietly  in 
one  position  for  two  or  three  minutes  before  opening  it.  This  is  in  order  to  insure 
that  the  side  of  the  yolk  which  contains  the  embryo  is  turned  uppermost.  After 
an  egg  is  disturbed  the  yolk  will  turn  and  resume  its  normal  position,  for  which 
but  a  short  time  is  necessary.  The  egg  may  now  be  held  in  one  hand,  the  shell 
cracked,  and  the  pieces  of  the  shell  above  the  yolk  be  removed  with  forceps,  mak- 
ing a  hole  about  an  inch  in  diameter.  The  inner  egg  membrane  may  be  removed 
with  the  shell.  If  any  of  the  white  of  the  egg  tends  to  overflow,  it  should  be 
immediately  snipped  off  with  a  pair  of  scissors,  otherwise  it  will  cause  the  yolk 
to  roll  over,  thus  concealing  the  embryo. 

The  embryo  and  germinal  area  are  now  to  be  examined  with  the  naked  eye 
or,  better,  with  a  hand  lens.  The  student  will  detect  very  easily  the  area  pellucida, 
which  lies  at  right  angles  to  the  long  axis  of  the  egg,  and  also  see  in  the  middle 
of  the  area  a  long  whitish  streak,  which  marks  the  anlage  of  the  embryo.  Around 
the  area  pellucida  can  be  seen  the  mottled  vascular  area  which  will  vary  in  ap- 
pearance according  as  the  development  of  the  blood-vessels  and  blood-islands  is 
more  or  less  advanced.  The  area  vasculosa  is ,  a  portion  of  the  larger  area  opaca 
which  merges  at  its  periphery  into  the  general  yolk.  In  embryos  of  the  second  half 
of  the  second  day,  thirty-six  to  forty-eight  hours,  the  contraction  of  the  heart  can 
be  readily  seen,  and  usually  the  outlines  of  the  head  of  the  embryo  may  be  made 
out.  The  germinal  area  is  now  to  be  separated  from  the  rest  of  the  yolk.  To 
accomplish  this,  plunge  one  blade  of  a  sharp  pair  of  scissors  into  the  yolk  a  little 
beyond  the  edge  of  the  vascular  area,  and  cut  rapidly  around  until  a  circular  inci- 
sion has  been  completed;  then  take  a  flat  .spatula  and  plunge  it  boldly  into  the 
yolk  at  a  depth  of  perhaps  an  eighth  of  an  inch  underneath  the  embryo.  Next 
lift  out  the  embryo  together  with  the  yolk  and  the  overlying  white  of  the  egg, 
steady  it  a  little  if  necessary  on  the  spatula  with  a  pair  of  forceps  or  needle,  and 
transfer  it  rapidly  to  the  dish  of  warm  salt  solution.  With  a  pair  of  fine  forceps 
the  edge  of  the  germinal  area  may  be  seized,  and  by  gentle  motion  it  may  be 
separated  from  the  mass  of  yolk  and  also  from  the  thin,  whitish,  overlying  mem- 
brane of  the  yolk,  and  at  the  same  time  from  so  much  of  the  white  of  the  egg 
as  may  have  been  carried  along.  As  one  becomes  more  practiced  in  these  opera- 
tions, it  is  not  difficult  to  remove  the  germinal  area  without  taking  much  yolk 
along  with  it. 

The  operation  may  be  modified  as  follows:  After  the  shell  is  opened  the  egg 
may  be  tilted  so  as  to  allow  the  white  to  run  off,  and  as  it  runs  over  the  edge 
it  is  snipped  through  with  the  scissors,  and  as  much  of  the  white  removed  as  is 


176  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

possible  in  this  way.  The  whole  egg  is  then  submerged  in  the  warm  salt  solution, 
an  incision  around  the  germinal  area  made  as  above,  and  the  embryo  floated  off 
from  the  yolk. 

Preservation  of  the  Embryo. — The  next  step,  after  the  embryo  has  been  removed 
from  the  yolk  and  lies  in  the  salt  solution,  is  to  put  a  glass  slide  in  the  salt  solu- 
tion and  carefully  float  the  embryo  and  germinal  area  upon  it,  and  then  remove 
them  together.  The  slide  is  now  to  be  laid  flat  on  the  table  and  the  germinal 
area  spread  out  carefully  upon  it.  In  this  operation  good  results  may  often  be 
obtained  by  allowing  a  few  drops  of  warm  salt  solution  to  fall  upon  the  center  of 
the  germinal  area.  The  currents  produced  by  the  falling  drops  will  be  sufficient 
to  spread  out  the  blastoderm  in  its  natural  form,*  and  at  the  same  time  to  wash 
away  any  superfluous  yolk  grains  that  may  be  adherent  to  the  preparation.  At 
this  stage  the  preparation  should  be  examined  by  the  student  with  a  low  power  of 
the  microscope,  as  described  below.  To  preserve  the  specimen,  four  or  five  drops 
of  Zenker's  fluid  are  allowed  to  fall  upon  the  specimen  gently  and  quietly  as  it 
lies  upon  the  glass  slide.  The  specimen  is  allowed  to  stand  for  about  ten  minutes 
and  is  then  transferred  to  a  dish  containing  a  larger  quantity  of  Zenker's  fluid. 
The  transfer  should  be  made  by  submerging  one  end  of  the  slide  in  the  dish  and 
floating  the  specimen  off.  In  from  two  to  four  hours  the  hardening  of  the  speci- 
mens will  be  completed.  They  must  then  be  washed  thoroughly  by  decanting  off 
the  Zenker's  fluid  and  replacing  it  with  water,  and  this  water  must  itself  be  replaced 
several  times  during  the  next  twenty-four  hours.  Further  treatment  of  the  specimen 
is  as  described  on  page  378. 

The  Making  of  Serial  Sections.— Specimens  are  best  colored  with  alum  cochineal 
in  toto.  They  are  then  imbedded  in  paraffin  and  cut  into  series.  The  most  useful 
sections  are  those  which  are  transverse  to  the  axis  of  the  spinal  cord.  They  should 
not  exceed  10/1  in  thickness. 

Embryo  Chick  with  Eight  Segments.     (About  twenty-eight  hours'  incubation.) 

The  following  description  is  almost  equally  applicable  to  embryos  with  six  or 
ten  segments. 

Examination  in  the  Fresh  State. — The  embryo  when  first  removed  from  the 
yolk  should  be  placed  in  a  staining-dish  with  a  small  quantity  of  normal  salt  solu- 
tion and  examined  with  a  low  power  of  the  microscope  as  a  transparent  object. 
The  specimen  as  a  whole  has  a  grayish  or  brownish  gray  tint.  Most  of  the  germi- 
nal area  is  dark,  the  transmission  of  light  being  stopped  by  the  numerous  yolk- 
grains  contained  in  the  entodermal  cells  (compare  page  64).  In  the  center  of  the 
germinal  area  the  transparent  area  pellucida  is  very  conspicuous,  and  has  an  edge 
which  is  quite  sharply  defined,  more  so  than  after  the  specimen  has  been  preserved. 
It  is  shaped  somewhat  like  an  elongated  pear,  the  broad  end  of  which  surrounds 


*  The  student  will  observe  that  the  fresh  blastoderm  is  verv  easily  distorted. 


EMBRYO  WITH  EIGHT  SEGMENTS. 


177 


the  cephalic  end  of  the  embryo  (Fig.  129).  It  should  be  noted  that  this  figure  was 
drawn  from  a  hardened,  and  not  from  a  fresh  specimen.  The  head  of  the  embryo 
lies  toward  the  large  end  of  the  area  pellucida  and  projects  freely  above  the  sur- 
face of  the  germinal  area.  Underneath  the  projecting  head  is  a  very  clear  area 
with  distinct  lateral  boundaries.  It  is  called  the  pro-amnion  and  contains  no  meso- 
derm  whatever.  Near  the  head  are  two  characteristic  areas,  one  on  each  side,  easily 
recognized  by  the  fact  that  the  surface  of  the  germinal  area  rises  like  a  dome  over 


FIG.  129. — CHICK  EMBRYO  AFTER  TWENTY-SEVEN  HOURS'  INCUBATION,  WITH  EIGHT  PRIMITIVE  SEGMENTS. 

ALCOHOLIC  SPECIMEN,    x  15  diams. 

each  space.  The  spaces  are  termed  the  amnio-cardiac  vesicles.  They  are  in  reality 
local  expansions  of  the  ccelom  which  cause  the  somatopleure,  or  upper  leaf  of  the 
germinal  area,  to  arch  upward  on  either  side  of  the  embryo.  By  the  study  of  cross- 
sections  (Fig.  136)  the  relations  may  be  clearly  understood.  The  posterior  limit  of 
the  head  is  marked  by  a  curving  line  (to  see  this  sharply  the  focus  must  be 
lowered)  the  cavity  of  which  faces  the  caudal  end  of  the  embryo.  This  line 
marks  the  position  of  the  fovea  cardiaca  and  from  it  the  fore-gut  extends  into  the 
head  of  the  embryo.  On  the  cephalic  side  of  the  fovea  and  underneath  the  fore- 
gut  the  heart  will  be  developed.  On  the  sides  of  the  fovea,  running  forward 


178  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

toward  the  median  line  of  the  embryo,  one  can  distinguish  two  darker  bands  which 
represent  the  beginning  of  the  formation  of  the  blood-vessels,  growing  in  from  the 
extra-embryonic  region  to  meet  in  the  median  line  of  the  embryo  and  participate 
in  the  formation  of  the  heart.  These  bands  are  the  anlages  of  the  omphalo-mesa- 
riac  veins.  Behind  the  fovea  appear  eight  pairs  of  rather  opaque  blocks  of  tissue, 
symmetrically  placed  right  and  left.  These  are  the  primitive  segments  and  are 
formed  exclusively  by  the  mesoderm.  The  first  pair  of  blocks  lie  a  short  distance 
behind  the  fovea  and  the  last  pair  a  short  distance  in  front  of  the  rhomboidal 
sinus  (compare  below).  When  new  segments  are  added  they  are  about  the  same 
size  as  those  previously  formed,  and  always  arise  at  the  caudal  end  of  the  series. 
The  growth  of  the  embryo  in  length  during  these  stages  depends  rather  upon  the 
multiplication  of  the  segments  than  upon  the  growth  of  the  single  segments.  The 
principal  axial  structure  is  the  anlage  of  the  central  nervous  system,  the  so-called 
medullary  groove,  already  partly  converted  into  a  medullary  canal;  for  at  this  stage 
it  is  closed  from  the  anterior  limit  of  the  head  to  a  variable  point  of  the  segmented 
region  of  the  embryo.  For  a  general  account  of  the  origin  of  the  medullary  groove 
from  the  ectoderm  see  page  67.  Chicks  with  eight  segments  vary  extremely  as  to 
the  extent  of  the  closure  of  the  groove.  The  line  of  .closure  can  be  readily  seen. 
It  is  somewhat  wavy  and  irregular  in  its  course,  and  the  closure  itself  is  some- 
what irregular,  so  that  we  may  find  one  or  several  points  where  the  closure  is  not 
yet  completed  although  it  is  complete  behind  and  in  front  of  these  points.  At  the 
anterior  extremity  of  the  head  the  closure  is  always  incomplete,  there,  being  an 
opening  there  which  persists  for  some  time  and  is  known  as  the  anterior  neuropore. 
Above  the  primitive  segments,  where  it  is  not  closed,  the  medullary  groove  has  its 
edges  close  together,  but  a  short  distance  behind  the  last  segment  the  groove  widens 
abruptly  and  fades  out  gradually.  This  widening  is  termed  the  rhamboidal  sinus. 
The  sinus  marks  the  caudal  limit  of  the  nervous  system  and  extends  so  as  to  em- 
brace the  cephalic  end  of  the  primitive  streak.  The  region  of  the  primitive  streak 
appears  quite  dark  by  transmitted  light,  owing  to  the  accumulation  of  cells  which 
belong  chiefly  to  the  mesoderm.  This  dark  appearance  extends  forward  and 
merges  into  a  dark  band  on  either  side,  which  runs  up  to  the  row  of  segments. 
The  dark  band  is  the  segmental  zone,  out  of  which  new  segments  are  differentiated. 
On  the  surface  of  the  primitive  streak  is  a  longitudinal  furrow,  the  primitive  groove, 
which  begins  just  within  the  rhomboidal  sinus  and  extends  backward,  often  bend- 
ing to  one  side  or  the  other,  usually  to  the  left.  The  groove  is  shallow  in  front, 
deeper  behind,  and  ends  quite  abruptly.  More  careful  examination  of  the  area 
opaca  shows  that  it  already  possesses  a  well-defined  area  vasculosa,  the  peripheral 
boundary  of  which  is  more  or  less  definitely  marked.  In  the  fresh  specimen  only 
traces  of  the  formation  of  the  blood-vessels  and  blood-islands  can  be  made  out. 

Examination  of  the  Specimen  after  Hardening. — The  specimen,  after  it  has  been 
hardened,  should  be  examined  under  the  microscope  in  water  or  alcohol;  and, 
again,  after  it  has  been  stained  it  should  be  cleared  in  oil  and  further' examined. 


EMBRYO  WITH  EIGHT  SEGMENTS. 


179 


This  will  enable  the  student  to  make  out  the  blood-islands  and  something  of  the 
blood-vessels  in  the  area  vasculosa,  and  also  the  shape  of  the  brain  which  (Fig. 
131)  has  expanded  widely  just  behind  the  neuropore;  the  lateral  expansions  are 
the  anlages  of  the  optic  vesicles  (Fig.  133).  The  remainder  of  the  brain  extends 
from  the  optic  enlargement  to  a  point  a  little  behind  the  fovea,  fov.  It  is  much 
wider  than  the  remaining  portion  of  the  medullary  canal;  it  tapers  from  the  optic 


FIG.  130. — EMBRYONIC  AREA  OF  A  RABBIT  WITH  EIGHT  SEGMENTS,  WITH  THE  PLACENTAL  AREA  PARTLY  TORN  OFF. 

X  15  diams.     (Drawn  by  T.  H.  Emerton.) 

vesicle  and  extends  backward.  One  cannot  yet  distinguish  in  it  positively  any 
subdivision  into  mid-brain  and  hind-brain.  On  the  contrary,  its  walls  are  often 
somewhat  irregularly  sinuous  and  vary  considerably  from  specimen  to  specimen. 

Comparison  with  a  Rabbit  Embryo. — In  the  ovum  of  the  mammalia  the  ecto- 
derm presents  a  modification  known  as  the  trophoderm.  In  the  rabbit  this  tropho- 
derm  is  developed  over  a  limited  region  which  is  called  the  placental  area  (Fig. 
130,  a. pi),  by  which  the  embryo  is  attached  to  the  wall  of  the  uterus.  When 
the  embryo  figured  was  removed,  a  portion  of  the  placental  area  remained  attached 
to  the  uterus,  hence  the  defect  shown  in  the  specimen.  The  vascular  area  is 


180  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

nearly  circular;  its  boundary  is  marked  by  a  well-defined  terminal  vessel,  v.t.  The 
nearly  straight  embryo  lies  in  the  center  and  exhibits  plainly  the  medullary  canal 
and  primitive  segments.  The  optic  evaginations  are  already  present.  The  head  is 
free;  on  its  under  side  the  heart  is  forming,  and  beneath  it  is  a  relatively  large 
and  conspicuous  pro-amnion,  pr.a.  Blood-vessels  are  present  over  the  area  vascu- 
losa,  but  not  yet  in  the  embryo.  It  will  be  seen,  therefore,  that  though  the  pro- 
portions differ  greatly  from  those  in  the  chick,  the  fundamental  relations  in  the 
rabbit  are  the  same  as  in  the  bird. 

Examination  of  the  Specimen  after  Staining. — After  the  chick  has  been  stained 
in  toto  it  should  be  cleared  in  oil  of  cloves,  or  other  suitable  fluid,  and  further 
examined  in  surface  views  with  low  powers  of  the  microscope.  For  this  purpose 
it  may  be  placed  in  a  small  shallow  staining  dish.  It  will  be  found  advantageous 
to  have  also  whole  chicks  with  their  area?  vasculosae  permanently  mounted  in 
Damar.  Embryos  up  to  about  forty-eight  hours'  incubation  are  readily  prepared  in 
this  way.  The  germinative  area  with  the  embryo  is  treated  like  an  ordinary  section. 
Specimens  thus  mounted  must  be  protected  from  the  pressure  of  the  cover-glass 
by  putting  under  two  opposite  edges  strips  of  paper  or,  better,  of  glass  to  support 
the  cover.  Strips  of  glass  as  needed  can  be  cut  from  broken  cover-glasses  with 
a  writing  diamond. 

Figure  131  represents  a  chick  with  eight  fully  formed  segments  stained  with 
alum  carmine  and  viewed  as  a  transparent  object. 

The  distribution  of  the  blood-islands  and  various  details  of  the  structure  of  the 
embryo,  which  in  the  fresh  specimen  are  obscure,  can  be  readily  observed  in  the 
cleared  preparation.  The  blood-islands,  Bl.is,  contain  crowded  young  blood-cells 
and  are  conspicuous  owing  to  the  intensity  with  which  they  are  stained.  They  are 
largest  and  most  numerous  in  the  posterior  part  of  the  area  opaca,  and  on  either 
side  become  gradually  smaller  and  farther  apart  toward  the  anterior  end  and  are 
absent  altogether  at  the  level  of  the  head.  A  few  small  islands  appear  in  the  area 
pellucida  around  the  caudal  end  of  the  embryo.  The  amnio-cardiac  vesicles,  A.c.v, 
are  marked  by  the  arching  up  of  the  surface  of  the  area  pellucida  on  both  sides 
of  the  head.  In  the  embryo,  the  segments,  Som.$,  the  walls  of  the  medullary  tube 
(brain,  Br,  and  spinal  cord,  Sp.c),  and  the  omphalo-mesaraic  veins,  V.om,  are 
sharply  defined.  The  first  segment  is  imperfectly  formed,  and  never  acquires  a 
full  development;  it,  together  with  segments  two,  three,  and  four,  are  called  the 
occipital  segments  because  they  enter  into  the  formation  of  the  occipital  region  of 
the  head  and  never  undergo  full  differentiation  like  the  other  segments.  In  mam- 
mals also  there  are  (probably  four)  occipital  segments.  The  fifth  segment  of  our 
chick  becomes  the  first  cervical  segment  of  the  adult.  The  medullary  tube,  Md, 
has  sharply  defined,  walls.  It  is  completely  closed  through  the  brain  region,  except 
at  the  anterior  neuropore.  At  its  cephalad  extremity  the  tube  has  expanded  later- 
ally to  form  the  optic  vesicles,  Op.V,  each  of  which  is  the  anlage  of  a  retina  and 
optic  nerve.  The  middle  region  of  the  tube,  between  the  optic  vesicles,  is  the  first 


EMBRYO  WITH  EIGHT  SEGMENTS. 


181 


cerebral  vesicle,  fore-brain  or  prosencephalon.  The  second  cerebral  vesicle,  mid-brain 
or  mesencephalon,  Br.2,  comprises  the  part  of  the  medullary  tube  immediately 
behind  the  optic  vesicles  and  includes  a  little  more  of  the  length  of  the  tube 
than  the  vesicles.  In  older  embryos  (14-20  segments)  it  becomes  clearly  marked 
off  by  constrictions  from  both  the  fore-  and  hind-brain.  The  third  cerebral 


G.  Pro.  am.     Np.         Op.  V.  a. 


Br. 


Br.3 


Fov 


Sp.  c. 


Sg.  z. 


V.  t. 


Pr.  s. 


FIG.  131. — CHICK  WITH  EIGHT  FULLY  FORMED  SEGMENTS,  STAINED  WITH  ALUM  COCHINEAL  AND  MOUNTED  IN 

DAMAR. 

a,  Outline  of  the  free  portion  of  the  head.  A.c.v,  Amnio-cardiac  vesicle.  A.p',  A.p",  Area  pellucida.  Bl.is, 
Blood-island.  Br.2,  Second  cerebral  vesicle.  Br.$,  Third  cerebral  vesicle.  Fov,  Fovea  cardiaca.  G, 
Post-optic  ganglionic  crest.  Md,  Medullary  groove.  Nch,  Notochord.  Np,  Anterior  neuropore.  Op.V, 
Optic  vesicle.  Ph,  Outline  of  pharynx.  Pro., am,  Pro-amniotic  area.  Pr.s,  Primitive  streak.  Sg.z,  Segmental 
zone  or  band  of  mesoderm,  from  which  new  segments  arise.  Som.^,  Third  somite.  Sp.c,  Spinal  cord.  V.om, 
Vena  omphalo-mesaraica.  V.t,  Vena  terminalis.  X  15  diams. 

vesicle,  hind-brain  or  rhombencephalon,  Br.T,,  is  as  long  as  the  other  two  and  has 
a  tapering  form;  it  becomes  the  cerebellum,  pons,  and.  me*dulla  oblongata  of 
the  adult.  The  cavity  of  the  brain  is  wide,  but  in  the  region  of  the  hind- 
brain  it  tapers  caudad  and  so  passes  over  into  the  narrow  cavity  of  the  spinal 


182  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

cord,  Sp.c.  In  this  specimen  the  open  medullary  groove,  Md,  begins  at  the  level  of 
the  fourth  segment.  The  outline,  a,  of  the  free  portion  of  the  head  is  sharply  marked, 
as  is  also  the  outline,  Ph,  of  the  pharynx  or  fore-gut,  which  opens  at  the  fovea 
cardiaca  into  the  general  sub-germinal  entodermic  cavity.  The  omphalo-mesaraic 
veins,  V.om,  can  be  traced  peripherally  to  their  junctions  with  the  vascular  net- 
work of  the  area  pellucida,  and  if  the  embryo  b,e  viewed  by  its  ventral  surface,  the 
two  veins  can  be  seen  to  unite,  beneath  the  fore-gut,  with  the  caudad  end  of  the 
heart.  The  notochord  can  be  seen  under  the  mid-  and  hind-brain,  as  a  narrow 
median  band  which  is  slightly  irregular,  and  also  very  clearly,  nch,  underneath 
the  rhomboidal  sinus;  it  fades  out  at  its  caudal  extremity  where  it  merges  into  the 
undiffer'entiated  tissue  of  the  primitive  streak.  A  band  of  cells,  G,  can  be  seen  on 
either  side  of  the  head,  extending  tailward  from  the  optic  vesicle.  This  band  is 
usually  designated  as  part  of  the  ganglionic  crest,  but  its  origin  and  fate  have 
not  yet  been  satisfactorily  elucidated. 

Longitudinal  Section  of  a  Chick. — In  order  to  facilitate  the  study  of  the  trans- 
verse sections  of  this  stage,  figure  132  is  inserted,  which  is  a  nearly  median  longi- 
tudinal section.  In  consequence  of  the  head  end,  H,  having  grown  .forward  above 
the  pro-amnion,  pro.a,  it  has  become  free  on  all  sides,  and  at  the  same  time  the 
entodermal  cavity  has  been  carried  forward  with  the  head,  making  the  so-called 
fore-gut  of  English  authors.  This  fore-gut  is  the  anlage  of  the  pharynx,  the  oeso- 
phagus, and  the  stomach.  Underneath  the  posterior  portion  of  the  fore-gut  there 
has  appeared  in  the  mesoderm'a  ccelomic  cavity,  />,'  which  serves  as  the  connection 
across  the  median  line  with  the  amnio-cardiac  vesicles  just  described  in  surface 
views.  We  can,  therefore,  distinguish  in  the  fore-gut  the  anterior  portion  from  the 
posterior  portion  which  overlies  the  ccelom.  This  coelom  is  the  anlage  of  the  peri- 
cardial  cavity*  The  anterior  division  of  the  fore-gut  forms  the  pharynx- proper.  It 
ends  blindly  in  front.  The  opening  of  the  fore-gut  into  the  general  entodermic 
cavity,  Ach,  is  termed  the  foiea  cardiaca,  fo.  At  the  posterior  end  of  the  embryo 
we  have  a  thickened  mass  of  cells  constituting  the  primitive  streak,  Pr.s.  The  line 
on  the  under  side  of  .the  figure  represents  the  entoderm,  and  the  space  underneath 
it  is  a  portion  of  the  primitive  entodermic  cavity. 

Study  of  Transverse  Sections.— Attention  should  be  directed,  first,  to  the  three 
germ-layers,  their  composition  and  their  rdles  in  the  production  of  organs;  second, 
to  the  exact  topographical  relations  of  the  various  organic  anlages,  because  these 
relations  are  fundamental  and  determine  the  anatomical  dispositions  in  the  adult. 
Before  beginning  the  detailed  study  of  the  sections,  the  student  should  have  a  clear 
conception  of  the  manner  in  which  the  free  head  of  the  embryo  merges  into  the 
embryonic  body  and  germinative  area.  Fifteen  figures  represent  as  many  cross- 
sections  of  an  embryo  chick  with  eight  fully  formed  segments,  and  the  ninth  seg- 
ment beginning.  The  drawings  are  uniformly  magnified  100  diameters.  There  are 
interpolated  figures  132,  138,  139,  147,  149  from  other  embryos  to  illustrate  certain 
details  with  higher  magnifications. 


EMBRYO  WITH  EIGHT  SEGMENTS. 


183 


Section  through  the  Optic  Vesicles  (Fig.  133). — The  section  is  oval,  the  head 
being  flattened  dorso-ventrally.  Its  outer  boundary  is  a  layer  of  cells,  EC,  consti- 
tuting the  ectoderm.  The  inner  and  outer  surfaces  of  the  ectoderm  are  marked  in 
the  section  by  distinct  lines.  With  higher  powers  the  ectodermal  nuclei  are  readily 
seen;  there  are  no  cell  boundaries,  although  the  protoplasm  is  gathered  into  columns 
and  strands  with  clear  spaces  between.  We  have  in  fact  to  deal  rather  with  a 
syncytium  than  with  a  layer  of  cells.  On  the  dorsal  side  the  ectoderm  shows  a 


pro.a 


FIG.  132.  —  LONGITUDINAL  SECTION  or  A  YOUNG  CHICK  EMBRYO. 

H,  Head.  Vd,  Anterior  portion  of  digestive  canal  (Vorderdarm).  mes,  Mesoderm.  fo,  Fovea  cardiaca.  p, 
Pericardial  coelom.  pro.a,  Pro-amnion.  Ach,  Entodermal  cavity,  in  life  bounded  below  by  the  yolk.  Pr.s^ 
Primitive  streak. 

thickening,  G.  If  this  be  followed  back  in  the  series  of  sections  it  will  be  found  to 
be  continuous  'both  with  the  ectoderm  and  with  an  internal  group  of  cells  alongside 
the  mid-brain  (Fig.  134,  G).  We  shall  return  to  the  consideration  of  the  group  in» 
question  in  connection  with  the  description  of  the  next  figure.  In  the  mid-dorsal 
line  the  ectoderm  from  each  side  reaches  the  anterior  neuropore,  Np,  which  is  still 
open,  and  is  reflected  inward  to  form  the  thicker  wall,  Md,  of  the  medullary  tube, 
here  widely  expanded  to  form  the  optic  vesicles,  Op.  The  outer  ectoderm,  EC,  and 


FIG.  133. — SECTION  OF  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.     TRANSVERSE  SERIES  642,  SECTION  21. 
EC,  Ectoderm.     G,  Ganglionic  thickening.     Md,  Wall  of  medullary  tube.     Np,  Neuropore.      Op,  Optic  vesicle, 

X  ioo  diameters. 

inner  ectoderm,  Md,  are  everywhere  in  contact  with  one  another,  so  that  in  the 
whole  section  there  is  but  a  single  germ-layer,  the  outer.  Soon  the  middle  germ- 
layer  will  penetrate  between  the  two  sheets  of  ectoderm,  and  permanently  obliterate 
the  primitive  relations. 

Section  through  the  Mid-brain  (Fig.  134). — The  section  of  the  head  is  oval,  and 
bounded  everywhere  by  the  ectoderm,  EC,  or  as  it  may  now  be  called,  the  epider- 
mis. The  head  is  completely  free,  but  underneath  lie  the  layers  of  the  germinal 


184 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


area.  Immediately  below  the  head  is  the  pro-amniotic  area  -(Pro.am)  which  con- 
sists of  only  two  very  thin  layers  of  cells,  the  upper  ectoderm,  the  inner  entoderm. 
By  following  through  the  series  of  sections  it  is  easy  to  satisfy  oneself  that  the  two 
layers  of  the  pro-amnion  are  directly  continuous  with  the  Irke-named  layers  of  the 
embryo  proper.  At  a  little  distance  from  the  head,  the  lateral  limit  of  the  pro- 
amnion  appears,  being  marked  by  the  appearance  of  the  mesoderm  between  the  other 
two  germ-layers.  The  edge  of  the  mesoderm  is  sharply  defined.  The  ectoderm  has 
formed  also  the  thick  wall,  Md,  of  the  medullary  tube,  which  at  this  point  is 
completely  closed  and  has  lost  its  connection  with  the  epidermis.  There  are  no 
distinct  cell  boundaries  anywhere  in  the  walls  of  the  medullary  tube  at  this  stage. 
The  nuclei  are  oval,  each  with  its  long  axis  more  or  less  nearly  vertical  to  the 
surface  of  the  tube.  Mitotic  figures  are  frequent  and  occur  always  near  the  inner 


mes.G. 


Md. 


Ao.d.     Ph.        a.       Ao.v 


FIG.  134. — SECTION  OF  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.     TRANSVERSE  SERIES  642,  SECTION  86. 
a,  Ventral  thickening  of  ectoderm  (part  of  oral  plate).     Ao.d,  Dorsal  aorta.     A o.v,  Ventral  aorta.    ,Ec,  Ectoderm. 
G,  Ganglionic  crest.     Md,  Wall  of  medullary  tube,  mid-brain,     mes,  Mesenchyma.     Ph,  Fore-gut.     Pro.am, 
Pro-amniotic  area.      X  100  diams. 

or  free  surface  of  the  medullary  wall;  in  other  words,  next  the  cerebral  cavity. 
The  microscopic  structure  of  the  tube  is  similar  throughout  its  whole  extent.  Under- 
neath the  brain  is  the  fore-gut,  Ph,  somewhat  crescentic  in  cross-section,  and  formed 
of  a  single  layer  of  epithelium,  the  entoderm,  which  is  thinner  on  the  dorsal,  thicker 
on  the  ventral  side  of  the  fore-gut,  a  difference  which  becomes  more  marked  in 
later  stages.  In  the  median  ventral  area  the  entoderm  is  somewhat  thickened  and 
adjoins  a  similar  thickening  ,  a,  of  the  underlying  ectoderm.  The  two  thickenings 
are  beginning  to  unite  at  present,  but  are  still  distinct  and  easily  break  apart. 
Very  soon,  however,  they  fuse  into  a  single  lamina,  which  is  known  as  the  oral 
plate  and  in  which  all  trace  of  the  double  origin  is  lost.  The  ectodermal  thicken- 
ing, a,  is  depressed  below  the  level  of  the  ventral  surface  of  the  head.  By  the  up- 
growth of  the  tissues  around  it,  the  depression  is  increased,  until  in  later  stages  it 
appears  as  a  deep  invagination,  lined  by  ectoderm,  and  the  floor  of  which  is  formed 
by  the  oral  plate.  The  invagination  is  termed  the  stomodcEum^.  and  is  destined  to 
form  a  large  part  of  the  mouth.  The  oral  plate  soon  undergoes  autolysis,  and  by 


EMBRYO  WITH  EIGHT  SEGMENTS.  185 

its  own  disappearance  creates  the  oral  opening  of  the  fore-gut.  Close  to  the  fore-gut 
lie  four  blood-vessels,  two  above  and  two  below,  the  dorsal,  Ao.d,  and  ventral, 
Ao.v,  aortae,  respectively.  The  dorsal  vessels  are  much  the  larger.  If  the  series 
of  sections  be  followed  through  cephalad  the  ventral  aorta  will  be  found,  before 
the  tip  of  the  fore-gut  is  reached,  to  bend  dorsalward  and  join  the  dorsal  aorta  of 
the  same  side.  If  the  series  Tbe  followed  through  in  the  caudad  direction,  it  will 
be  observed  that  the  two  ventral  aortae  draw  toward  the  median  line  until  they 
meet  and  unite  in  a  single  trunk,  the  main  aorta,  which  is  continuous  with  the 
heart  (Fig.  135,  Hf).  It  is  thus  learned  that  the  blood  leaves  the  heart  at  its  cepha- 
lic end  by  a  single  channel,  which  soon  divides;  the  branches  curve  upward  and 
pass  to  the  dorsal  side  of  the  pharynx,  forming  two  dorsal  channels  conducting 
the  blood-stream  backward.  The  blood-vessels  consist  each  of  a  very  thin  layer 
of  cells,  epithelial  in  character  and  termed  endothelium;  the  nuclei  are  flattened 
and  therefore  appear  oval  in  section.  All  the  remainder  of  the  section  is  occupied 
by  loosely  scattered  cells,  which  are  of  two  sorts:  first,  those  marked  mes,  which 
till  the  ventral  and  lateral  regions,  and  constitute  the  true  mesenchyma;  the  mesen- 
chymal  cells  have  nuclei  with  small  amounts  of  protoplasm  around  them,  and 
strands  of  protoplasm  connect  the  cells  together;  there  are  no  cell  boundaries; 
the-  t'ssue  might  be  described  as  an  irregular  reticulum  with  nucleated  nodes; 
second,  those  cells  marked  G,  which  form  two  lateral  groups  on  the  dorsal  side 
adjoining  the  mid-brain;  these  groups  have  been  named  the  ganglionic  crests  by 
some  writers,  mesectoderm  by  others.  The  cells  in  question  resemble  those  of  the 
true  mesenchyma,  but  have  more  protoplasm  around  the  nuclei  and  appear 
therefore  more  deeply  stained  than  the  mesenchyma  proper.  If  the  cells  of  the 
crest  be  followed  dorsally  they  will  be  seen  to  form  a  narrow  band  which  joins  the 
ectoderm  near  the  median  line,  and  by  following  the  sections  headward,  the  crests 
will  be  fouHd  to  merge  with  ectodermal  thickenings  (Fig.  133,  G).  From  these 
relations  it  has  been  inferred  that  the  crest  on  each  side  arises  from  a  local  pro- 
liferation of  the  ectoderm.  The  crest  is  easily  seen  in  surface  views  of  stained 
chicks  (Fig.  131,  G).  Two  principal  views  as  to  the  future  of  the  crest  cells  of  the 
mid-brain  region  have  been  brought  forward:  first,  that  they  are  true  ganglionic 
anlages,  which  disappear  by  autolysis;  second,  that  they  are  converted  into  true 
mesenchyma. 

Section  through  the  Hind-brain  (Fig.  135). — The  head  is  no  longer  free,  but 
fuses  laterally  with  the  layers  of  the  germinal  area;  hence  the  ectoderm,  EC,  instead 
of  bending  over  on  to  the  ventral  side,  bends  in  the  opposite  direction— away  from  the 
embryo.  The  mesoderm  stretches  across  the  median  line  under  the  embryo.  There 
is  a  large  space,  Coe,  in  the  mesoderm;  the  space  is  part  of  the  primitive  body-cavity 
or  ccelom;  it  extends  completely  across  the  embryo  and  out  into  the  germinal  area 
on  each  side.  The  ccelom  is  everywhere  bounded  by  a  thin  epithelial  layer,  msth, 
the  mesothelium,  which  at  this  stage  resembles  an  endothelium  as  seen  in  section; 
it  forms  one  part  of  the  mesoderm,  the  bulky  mesenchyma  forming  the  other  part. 


186 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


Below  the  ccelom  is  another  cavity,  Pro.am,  that  of  the  pro-amnion,  lined  by  ecto- 
derm and  opening  anteriorly.  This  structure  is  not  further  dealt  with  here,  partly 
because  its  history  is  complicated,  partly  because  it  does  not  occur  in  the  human 
embryo.  The  ventral  aortae  (Fig.  134,  Ao.v)  have  united  into  a  single  blood-chan- 
nel, Ht,  which  we  can  identify  as  the  blood-channel  of  the  heart;  it  is  called  the 
endothelial  heart.  The  mesothelium,  msth,  on  the  dorsal  side  of  the  ccelom  forms 
a  protuberant  fold,  the  mesothelial  heart,  which  surrounds  the  inner  vascular  space. 
The  two  heart-walls  are  some  distance  apart.  The  inner  heart  produces  only  the 
lining  endothelium  of  the  adult  organ,  the  mesothelial  heart  produces  all  its  muscu- 
lar and  connective-tissue  components,  and  also  the  pericardium.  The  difference 


mes.      Ao.d. 


Aid. 


Coe.        Ent. 


nth.  Pro.am.     msth. 


FIG.  135. — SECTION  OF  CHICK  WITH  EIGHT  SEGMENTS.     TRANSVERSE  SERIES  86,  -SECTION  86. 
Ao.d,  Dorsal  aorta.     Coe,  Ccelom.     EC,  Ectoderm.     Ent,  Entoderm.     G.  Ganglionic  crest.     Ht,  Aortic  end  of 
the   endothelial    heart,     nch,  Notochord.     Md,  Hind-brain,     mes,  Mesenchyma.     msth,  Mesothelium.     Ph, 
Fore-gut.     Pro.am,  Pro-amnion.      X  100  diams. 

in  the  form  of  the  cross-sections  of  the  hind-brain,  Md,  mid-brain,  and  fore-brain 
should  be  noted.  Between  the  brain  and  fore-gut,  and  touching  both,  lies  the  small 
notochord,  nch,  with  a  sharp  outline.  The  ganglionic  crest,  G,  of  the  mid-brain  is 
still  traceable,  but  occupies  a  much  smaller  area,  than  in  figure  134.  In  the  mid- 
dorsal  line  the  crest,  G,  the  epidermis,  EC,  and  the  medullary  tube,  Md,  are  fused 
together.  In  correspondence  with  the  reduction  of  the  crest,  the  area  occupied  by 
the  true  mesenchyma,  mes,  is  increased. 

Section  through  the  Cephalic  (Aortic)  End  of  the  Heart  (Fig.  136). — The  general 
topography  is  similar  to  that  of  figure  135.  The  most  striking  differences  are,  that 
in  the  present  section  the  heart  is  very  much  larger,  is  bent  to  the  right  of  the 
embryo  (the  left  of  the  figure),  and  has  a  narrow  connection  (mesocardium)  with 
the  floor  of  the  pharynx;  that  the  ccelom  is  much  expanded  to  form  the  amnio- 
cardiac  vesicles,  A.c.v,  one  on  each  side,  which  are  continuous  with  one  another 
On  the  ventral  side  of  the  heart;  that  the  lips  of  the  medullary  tube  are  in  contact 
at  c,  but  have  not  actually  fused;  and  that  there  is  no  pro-amnion,  because  it  does 
not  extend  so  far  back  under  the  embryo.  The  following  details  should  be  ob- 


'EMBRYO  WITH  EIGHT  SEGMENTS. 


187 


served:  The '  epidermis,  EC,  of  the  embryo  is  thickened  and  fits  closely  against  the 
dorsal  portion  of  the  hind-brain,  with  which  it  is  actually  fused  in  the  median  line. 
The  ganglionic  crest  is  represented  only  by  a  few  more  lightly  stained  cells  at  the 
junction  of  the  epidermis  with  the  medullary  wall,  M d,  but  is  much  more  developed 
in  nearby  'Sections,  both  cephalad  and  caudad.  The  fore-gut,  Ph,  is  very  wide,  the 
entoderm  on  its  dorsal  side  is  very  thin,  but  grows  thicker  toward  the  lateral 
boundaries  of  the  gut,  and  is  thickest  in  the  mid-ventral  line,  where  it  forms  a 
shallow  median  groove;  the  nuclei  in  this  groove  are  all  next  the  external  surface 
of  the  entoderm.  The  mesothelium,  msth,  is  a  thin  layer,  which  above  the 
amnio-cardiac  vesicles  enters  into  the  formation  of  the  somatopleure  or  true  body 


msth.        mes.         Ao.d. 


A.c.v  Endo.     m.Ht.     nch         Ph.         Spl. 

FIG.  136. — SECTION  OF  A  CHICK  EMBRYO  WITH  EIJGHT  SEGMENTS.    TRANSVERSE  SERIES  642,  SECTION  114. 
A.c.v,  Amnio-cardiac  vesicle.     Ao.d,  Dorsal  aorta,     c,  Line  of  Closure  of  the  medullary  canal.     EC,  Ectoderm.. 
Endo,  Endothelial  heart.     Md,  Wall  of  hind-brain,     mes,  Mesenchyma.     m.Ht,  Mesothelial  heart,     msth, 
Mesothelium.     nch,  Notochord.     Ph,  Fore-gut.     Spl,  Splanchnopleure.      X  IO°  diams. 

wall.  It  is  not  sharply  separated  from  the  mesenchyma,  mes,  as  can  be  very  well 
seen  in  the  part  of  the  layer  underneath  the  pharynx.  When  the  heart  is  reached 
the  mesoderm  forms  a  wide  duplicature,  m.Ht,  }the  mesothelial  heart,  which  is  a 
layer  of  much  greater  thickness  than  the  mesothelium  proper,  and  which  offers  the 
important  characteristic  that  it  shows  no  differentiation  into  mesothelium  and 
mesenchyma.  -Between  the  mesothelial  heart-tube  and  the  endothelial,  Endo,  there 
is  a  wide  space  which  contains  no  visible  structures,  hence  we  assume  that  the 
two  cardiac  tubes  are  kept  apart  by  fluid  only.  Beneath  the  ccelom  (amnio-cardiac 
vesicles,  A.c.v)  is  the  Splanchnopleure,  Spl,  which  has  two  thin  layers:  the  upper 
is  mesoderm,  the  lower  entoderm.  The  mesoderm  has  numerous  nuclei,  and  if 
followed  out  laterally  to  the  area  opaca  will  be  found  affixed  to  blood-vessels  and 
blood-islands,  which  together  constitute  the  angioblast  or  anlage  of  the  vascular  sys- 
tem. It  can  be  observed  in  most  places  readily  that  the  angioblast  lies  beneath  the 
mesoderm  proper  and  is  distinct  from  it.  The  entoderm  has  few  nuclei  and  in 
the  area  pellucida  is  very  thin,  but  where  it  passes  to  the  area  opaca  it  gradually 
but  rapidly  thickens,  and  .is  composed  of  very  large  columnar  cells  (compare  Fig.  30) 


188 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


with    large    vacuoles,   left    by   yolk    masses  which    the    cells   have   digested;    toward 
the  periphery  vacuoles  with  partly  absorbed  yolk  may  be  found. 

Section  through  the  Venous  End  of  the  Heart  (Fig.  137).— The  relations  differ 
but  little  from  those  in  figure  136  except  for  the  heart  and  the  ganglionic  crest. 
The  heart  shows  its  bend  toward  the  left  side  (the  right  in  the  figure),  and  in 
this  bend  both  the  endothelial,  Endo,  and  the  mesothelial  portions,  m.Ht,  partici- 
pate. The  mesocardium,  x,  is  clearly  recognizable,  and  comprises  two  mesodermic 
lamina.  By  it  the  heart  is  suspended  throughout  its  length  from  the  ventral  wall 
of  the  fore-gut.  The  mesocardium  soon  disappears,  and  the  mesothelial  heart  there- 
upon becomes  closed  dorsally,  and  is  attached  only  by  its  aortic  and  venous  ends 
to  the  neighboring  tissues.  A  strand  of  cells  from  the  endothelial  heart  passes 


Som.        EC. 


Ao.D        G. 


Md. 


Acv. 


m.Ht. 


x       Endo. 


Ph. 


FIG.  137. — SECTION  OF  A  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.     TRANSVERSE  SERIES  642,  SECTION  130. 
Acv,  Ammo-cardiac  vesicles.     Ao.D,  Dorsal  aorta.     EC,  Epidermis.     Endo,  Endothelial  heart.     G,  Ganglionic 
crest.     Md,   Medullary    tube    (Hind-brain),     mes,  Mesenchyma.     m.Ht,  Mesothelial    heart.     Ph,  Fore-gut. 
Som,  Somatopleure.     Spl,  Splanchnopleure.     x,  Mesocardium.      X  100  diams. 

through  the  mesocardium  and  joins  the  entoderm.  The  significance  of  this  junc- 
tion is  not  clear.  The  ganglionic  crest,  G,  is  very  distinct;  it  is  overlaid  by  a  thin 
lamina  of  the  epidermis,  and  is  in  texture  quite  unlike  the  brain-wall  proper,  Md. 
The  cells  composing  it  are  considerably  individualized  and  somewhat  separated  from 
one  another  by  clear  spaces. 

Figure  138  represents  a  section  somewhat  more  highly  magnified  through  the 
heart  anlage  of  a  slightly  younger  embryo.  The  medullary  groove,  Md,  is  not 
closed.  The  ccelom  does  not  yet  extend  across  the  median  line,  but  there  is  only 
a  thin  partition  separating  the  amnio-cardiac  vesicles,  Am.ves,  from  one  another. 
The  mesothelial  heart,  msth,  is  a  relatively  thick  layer,  thrown  into  irregular  folds. 
The  endothelial  heart  is  represented  only  by  a  few  scattered  angioblastic  cells, 
Endo,  which  as  yet  show  no  definite  order. 

The  further  development  of  the  heart  may  be  understood  by  the  examination 
of  a  somewhat  older  stage  (Fig.  139).  As  shown  in  the  illustration,  the  mesothe- 
lium  has  become  very  protuberant,  m.ht,  in  the  median  line  underneath  the  fore- 
gut,  Ph.  On  either  side  it  rapidly  thins  out,  ,msth.  In  the  protuberant  fold  we 


EMBRYO  WITH  EIGHT  SEGMENTS. 


189 


can  recognize  the  future  muscular  heart,  as  it  is  sometimes  called.  The  few  cells 
above  described  (Fig.  138,  Endo)  have  increased  considerably  in  number  and  have 
joined  themselves  together  in  such  a  manner  as  to  indicate  clearly  the  formation  of 


FIG.  138. — SECTION  OF  A  CHICK  EMBRYO  WITH  SEVEN  SEGMENTS.     TRANSVERSE  SERIES  510,  SECTION  184. 

Am.ves,  Amnio-cardiac  vesicle.     EC,  Ectoderm.     Endo,  Cells  forming  the  anlage  of  the  endothelialjheart.     Md, 

Hind-brain,     mes,    Mesenchyma.     msth,    Anlage    of     mesothelial    heart.     Ph,    Fore-gut. 


m.ht 


pro.  am 


FIG.  139. — CHICK  EMBRYO,  TRANSVERSE  SECTION  ACROSS  THE  ANLAGE  OF  THE  HEART  IN  A  STAGE  SLIGHTLY  MORE 

ADVANCED  THAN  FIGURE  138. 

Md,  Wall  of  medullary  tube,     nch,  Notochord.     msth,  Mesothelium.     Ph,  Pharynx,     pro.am.,  Tip  of  pro-amnion. 

en.ht,  Endothelial  heart,     m.ht,  Muscular  heart. 

the  endothelial  heart  (Fig.  139,  en.ht).  At  first  the  cells  are  irregularly  disposed  and 
have  several  irregular  cavities  between  them,  which  soon  fuse  so  as  to  form  two 
main  cavities  running  longitudinally.  As  the  two  cavities  enlarge  they  meet  in  the 
median  line  and  remain  separated  at  first  by  a  wall  of  two  layers  of  endothelium. 


190 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


This  wall  soon  •  breaks  through,  and  there  results  a  single  median  tube  of  endo- 
thelium  which  presently  appears  to  be  connected  with  the  mesothelium,  m.ht, 
by  long  cell-processes  across  the  wide  intervening  space.  The  heart  is  now  a 
double  tube  connected  by  the  mesothelium  with  the  tissues  above. 

Section  through  the  Wall  of  the  Fovea  cardiaca  (Fig.  140). — Underneath  the  whole 
of  the  embryo  and  the  germinative  area  is  the  extensive  archenteric  cavity,  bounded 
above  by  the  cellular  entoderm  of  the  embryo  and  of  the  splanchnopleure.  The 
archenteron  includes  both  the  intestinal  cavity  of  the  embryo  and  the  cavity  of  the 
yolk-sac,  and  accordingly  its  lower  floor  is  the  mass  of  yolk.  Into  the  head  of 
the  embryo  runs  the  closed  prolongation  of  the  archenteron,  which  we  have  studied 
as  the  fore-gut.  The  posterior  opening  of  the  fore-gut  is  known  as  the  fovea 
cardiaca.  The  manner  in  which  the  entoderm  at  the  fovea  bends  ventralward  to 


Sont. 


Ao.d.       Md. 


EC. 


Spl.  COB.  msth.       Ent.  Ph.       V.om. 

FIG.  140. — SECTION  OF  A  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.     TRANSVERSE  SERIES  642,  SECTION  149. 

Ao.D,   Dorsal   aorta.     Cos,   Coelom.     EC,   Epidermis.      Ent,   Entoderm.     G,  Ganglionic  crest.     Md,  Hind-brain. 

msth,  Mesoderm  of   the   septum  transversum.     Ph,  Fore-gut.    Som,  Somatopleure.    Spl,   Splanchnopleure. 

V.om,  Omphalo-mesaraic  veins.     X  100  diams. 

pass  from  the  fore-gut  on  to  the  splanchnopleure  may  be  readily  understood  from 
figure  139.  The  present  section  passes  through  the  nearly  vertical  wall  of  entoderm, 
Ent,  at  the  fovea.  From  this  wall  the  anlage  of  the  liver  will  arise.  THe  omphalo-. 
mesaraic  veins,  V.om,  pass  by  it  to  join  the  caudal  or  venous  end  of  the  heart. 
The  fore-gut,  Ph,  is  still  closed  on  its  ventral  side.  The  veins  are  in  the  splanch- 
nopleure, and,  being  cut  obliquely,  appear  somewhat  elongated.  They"  each  cause  a 
1  protuberance  of  the  splanchnopleure  toward  the  ccelom,  Coe.  The  protuberance 
is  covered  by  a  thick  dense  layer  of  mesoderm,  Msth,  which  forms  an  arch  over 
the  vein,  so  as  to  leave  a  clear  space  between  it  and  the  endothelium  of  the  vein. 
The  two  protuberances  constitute  the  anlage  of  the  septum  transversum,  which  is 
itself  the  anlage  of  the  diaphragm.  The  region  cephalad  of  the  septum  is  the 
cervico-thorax;  the  region  caudad,  the  future  abdomen.  If  the  series  of  sections  be 
followed  headward,  the  veins  can  be  traced  to  their  union  with  the  heart  in  the 
median  line.  If  the  series  be  followed  tailward,  the  veins  can  be  traced  out  into 
the  area  pellucida,  where  they  branch.  The  hind-brain,  Md,  is  of  smaller  diameter 
than  in  the  previous  sections.  The  epidermis,  EC,  is  closely  fitted  against  the  dorsal 


EMBRYO  WITH  EIGHT  SEGMENTS. 


191 


half  of  the  medullary  tube  and  fuses  in  the  mid-dorsal  line  with  the  ganglionic 
crest,  G,  which  in  its  turn  fuses  with  the  medullary  walh  The  crest,  though  not 
large  or  conspicuous,  can  be  distinguished  readily.  The  mesenchyma,  is  clearly 
differentiated  only  above  the  fore-gut,  and  on  either  side  just  beyond  the  lateral 
boundary  of  the  fore-gut  it  fuses  with  the  mesotheliurn.  > 


Som. 


Ao.D.   Md. 


G. 


mes.      EC. 


Spl.  Ent.       nch.      '     a.  V.om. 

FIG.  141. — SECTION  OF  A  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.    TRANSVERSE  SERIES  642,  SECTION  -144. 
a,  Groove  corresponding  to  the  prolongation  of  the  lateral  portion  of  the  fore-gut.     Ao.D,  Dorsal  aorta.     EC, 
Epidermis.     Ent,     Entoderm.      G,    Ganglionic     crest.      Md,    Wall    of    Hind-brain,     mes,    Mesenchyma. 
nch,    Notochord.    Som,    Somatopleure.    Spl,   Splanchnopleure.     V.om,    Omphalo-mesaraic  veins.     X  100 
diams. 

Section,  behind  the  Fovea  cardiaca  and  in-  Front  of  the  First  Segment  (Fig.  141).— 
The  closed  entodermal  cavity  of  the  embryo  has  become  open  and  communicates 
freely  with  tHe  yolk-cavity.  The  omphalo-mesaraic  veins  occupy  a  more  lateral 
position.  Otherwise  the  section  differs  so  little  from  figure  140,  that  it  does  not  call 
for  special  description.  * 


Som. 


EC. 


Md. 


Spl.         Cce.  Ao.D.      nch.  Ent.      Msth. 

.  FIG.  142.— SECTION  OF  A  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.     TRANSVERSE  SERIES  642,  SECTION  162. 
Ao.D,   Dorsal   aorta.     Ch,   Ccelom.     EC,   Ectoderm.     Ent,   Entoderm.     G,   Anlage  of  Ganglionic  crest,     mes, 

Mesenchyma  of  the  intersegmental  cleft.     Msth,  Mesodermic  lining  of  the  coelom.     nch,  Notochord.     Som, 

Somatopleure.    Spl,  Splanchnopleure.      X  100  diams. 

Section  between  the  First  and  Second  Segments  (Fig.  142). — We  are  still  in  the 
region  of  the  hind-brain,  which  extends  to  the  fourth  or  last  occipital  segment. 
There  is  no  distinction  or  limit  between  the  embryonic  and  the  vitelline  divisions 
of  the  archenteron.  The  hind-brain,  Md,  is  oval  in  section;  on  its  dorsal  side  it 
fuses  with  the  ganglionic  crest,  G,  which  seems  now  rather  a  part  of  the  brain-wall 


192 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


than  a  separate  structure.  The  mid-ventral  wall  or  floor  of  the  hind-brain  is 
relatively  thin,  a  feature  which  marks  the  transition  to  the  spinal  cord,  which  al- 
ways has  a  thin  floor-plate.  The  notochord,  nch,  is  large  and  transversely  oval  in 
section.  The  two  dorsal  aortae,  Ao.D,  occupy  the  same  relative  positions  as  in  the 
previous  sections.  The  ccelom,  Coe,  is  a  comparatively  narrow  fissure,  but  can  be 
followed  laterally  far  out  into  the  area  op^aca.  It  is  bounded  above  and  below  by 
mesoderm,  Msth,  for  the  most  part  thin  and  of  a  loose  texture;  but  on  the  lower 
side  in  the  embryonic  region  the  mesoderm  forms  a,  broad  band,  the  cells  of  which 
are  densely  packed.  The  thick  mesodermic  band  is  continuous  with  that  which 
forms  the  covering  of  the  septum  trans versum  (Fig.  140,  Msth).  The  morphological 
significance  of  the  band  is  undetermined.  The  mesenchyma,  mes,  occupies  the 
space  between  the  hind-brain  and  ectoderm  on  the  one  hand  and  the  aorta  and 


EC. 


Som.m. 


Ent. 


Cos.       Ao.D.  nch. 


Mes. 


seg.      Spl.m. 

FIG.  143. — SECTION  OF  A  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.     TRANSVERSE  SERIES  642,  SECTION  180. 
Ao.D,  Dorsal  aorta.     Cos,  Crelom.     EC,  Ectoderm.     Ent,  Entoderm.     Md,  Hind-brain.     Mes,  Mesoderm.     nch, 
Notochord.     seg,  Mesodermic  segment  or  somite.     Som.m,  Somatic  mesoderm.     Spl.m,  Splanchnic  meso- 
derm.     X  100  diams.  • 

ccelom  on  the  other.  It  consists  of  loosely  scattered  cells,  connected  with  one 
another  by  strands  of  protoplasm.  It  fuses  at  the  proximal  angle  of  the  coelom  with 
the  lining  mesoderm  thereof.  Longitudinal  sections  demonstrate  that  the  loose 
mesenchyma  occupies  only  the  narrow  space  between  two  segments,  and  that  it 
fuses  with  the  denser  tissue  of  both  adjacent  segments.  The  intersegmental  space 
is  not  a  cleft  but  a  partition  of  loose  mesenchyma. 

Sections  through  the  Third  Segment  (Fig.  143). — As  the  hind-brain  ends  at  the 
level  of  the  fourth  segment,  the  present  section  is  near  the  transition  from  the 
cephalic  to  the  cervical  region,  segment  5  being  the  first  cervical  segment.  Accord- 
ingly we  find  that  the  medullary  tube,  Md,  although  not  yet  closed,  has  in  cross- 
section  a  form  resembling  that  of  the  cord  at  this  stage.  The  ectoderm,  EC,  runs' 
from  the  lips  of  the  medullary  groove  as  a  layer  which  is  somewhat  thickened  over 
the  embryo,  but  becomes  very  thin  over  the  area  pellucida.  The  coelom,  Coe,  is  of 
small  dimensions,  although  irregular  clefts  in  the  mesoderm  of  the  germinative  area 
indicate  its  extension.  The  somatic  mesoderm,  Som.m,  is  a  thin  layer  above  the 
ccelom;  the  splanchnic  mesoderm  is  a  much  thicker  layer,  Spl.m,  below  the  ccelom. 
Both  mesodermic  layers  extend  beyond  the  coelom  toward  the  medullary  tube  to 
form  the  mesodermic  somite,  seg,  which  with  its  fellow  of  the  opposite  side  constitutes 


EM BR  YO  WI TH,  EIGH  T  SEGMEN  TS.  1 93 

a  complete  segment.  The  mesoderm  does  not  extend  across  the  median  line,  being 
blocked  by  the  notochord,  nch.  The  somite  is  bounded  mesially  by  the  medullary 
tube  and  notochord,  dorsally  by  the  ectoderm,  ventrally  by  the  aorta  and  entoderm. 
Its  cells  are  so  arranged  as  to  make  a  dorsal,  a  mesial,  and  a  ventral  wall,  and 
a  core  of  cells  more  loosely  grouped.  Careful  study  of  the  segments  in  various 
stages  has  led  to  the  conclusion  that  the  core  belongs  to  the  ventral  wall.  The 
line  of  contact  between  the  dorsal  wall  and  the  core  is  accordingly  the  potential 
prolongation  of  the  ccelom,  and  in  certain  embryos  there  is  a  ccelomatic  space  pres- 
ent in  the  position  indicated.  The  somite  has  a  broader  part  toward  the  medullary 
wall  and  a  narrower  part  toward  the  main  coelom.  When  a  segment  undergoes  its 
full  development,  the  narrow  part  forms  a  separate  structure,  the  nephrotome. 

Section  through  the  Segmental  Zones  (Fig.  144). — The  medullary  groove,  Md, 
is  not  closed,  but  is  deep  and  narrow,  its  dorsal  lips  nearly  in  contact  with  one 
another.  The  embryonic  ectoderm,  EC,  is  slightly  thickened.  The  entoderm  is 


Mes.  Seg.  z.         Md 


Ent.  Ao.  nch.  Cce.      Ve. 

FIG.  144. — SECTION  OF  A  CHICK  J^MBRYO  WITH  EIGHT  SEGMENTS.     TRANSVERSE  SERIES  642,  SECTION  267. 
Ao,    Dorsal  aorta  passing  from  the  embryo  to  the  area  vasculosa.     Cos,  Beginning  of  coelomatic  cavity.     EC, 
Ectoderm.     Ent,  Entoderm.     Md,  Medullary  groove.     Mes,    Mesoderm.      nch,    Notochord.     Seg.  z,  Seg- 
mental zone  of  the  mesoderm.     Ve,   Blood-vesseJ  of  the  area  vasculosa  lying  below  the  mesoderm  proper. 
X  100  diams. 

a  thin  layer  which  on  one  side,  Ent,  shows  the  beginning  of  the  thickening  char- 
acteristic of  the  area  opaca.  The  notochord,  nch,  is  nearly  circular  in  section  and 
is  larger  here  than  nearer  the  head.  The  dorsal  aortae,  Ao,  have  left  their  posi- 
tion and  are  passing  outward  to  ramify  upon  the  area  pellucida.  It  is  thus  evi- 
dent that  the  distribution  of  the  blood  from  the  heart  to  the  area  vasculosa  takes 
place  considerably  caudad  of  the  veins  which  collect  the  blood  and  return  it  to  the 
heart.  The  blood-vessels,  Ao.Ve,  lie  between  the  mesoderm  proper  and  the  ento- 
derm, and  constitute  with  the  associated  blood-islands  the  angioblast.  The  situa- 
tion of  the  angioblast  in  early  stages  is  typical  for  all  birds  and  also  for  mammals. 
The  mesoderm,  Mes,  has  only  irregular  spaces,  Coe,  which  by  their  expansion  and 
fusion  will  give  rise  to  the  continuous  coelom.  On  either  side  of  the  medullary 
groove,  the  mesoderm  forms  a  thickened  mass,  the  segmental  zone,  Seg.z,  which 
is  markedly  constricted  where  it  joins  the  lateral  mesoderm.  Out  of  the  constricted 
area  the  nephrotomes  are  differentiated.  The  somites  arise  by  transverse  cleavage 
of  the  segmental  zone,  each  new  somite  being  formed  immediately  caudad  to  the 
last-formed  somite.  The  so-called  cleavage  depends  upon  a  great  loosening  of  the 


194 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


mesenchyma,  as  shown  in  figure  142,  and  not  upon  the  development  of  an  actual 
fissure  or  cell-less  space.  If  the  series  of  sections  be  followed  toward  the  tail,  the 
segmental  zone  will  be  found  to  fade  out  gradually. 

Section  through  the  Open  Medullary  Groove  (Fig.  145). — As  we  have  seen  in 
following  the  series  of  sections,  the  farther  tailward  we  pass  the  less  advanced  is 
the  development.  In  the  present  section  (Fig.  145),  we  find  that  only  the  notochord, 
nch,  is  separated  from  its  germ -layer.  The  three  germ -layers,  the  ectoderm,  EC, 


Aid. 


Ales. 


Ent.  nch.  Ve. 

FIG.  145. — SECTION  OF  A  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.  TRANSVERSE  SERIES  642,   SECTION  314. 

EC,    Ectoderm.     Ent,    Entoderm.     Md,    Medullary    groove.  -  Mes,  Mesoderm.     nch,    Notochord.     Ve,  Blood- 
vessels.     X  loo  diams. 

mesoderm,  Mes,  and  entoderm,  Ent,  are  merely  laminae  of  undifferentiated  cells, 
the  ectoderm  alone  showing  a  modification  where  it  forms  the  wall  of  the  medul- 
lary groove,  .Md.  The  angioblast  or  layer  of  blood-vessels,  Ve,  forms  a  well- 
defined  separate  anlage,  which  is  clearly  distinct  from  both  mesoderm'and  entoderm. 
The  notochord,  nch,  is  of  large  size,  and  in  part  occupies  'a  notch  on  the  .under 
side  of  the  medullary  groove.  A  little  farther  caudad  the  notochord  fuses  with  the 
wall  of  the  overlying  medullary  groove,  and  still  farther  on  the  united  structures 
fuse  also  with  the  entoderm  (Fig.  146). 


Md 


Ales. 


Ent.  Pr.S. 

FIG.  146. — SECTION  OF  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.     SERIES  642,  SECTION  350. 
EC,    Ectoderm.     Ent,   Entoderm.     Md,   Medullary  groove.     Mes,   Mesoderm.     Pr.S,   Primitive   streak.      X.ioo 

diams. 

Section  through  the  Medullary  Groove,  near  its  Caudal  End  (Fig.  146). — The 
disposition  of  the  parts  is  somewhat  similar  to  that  in  figure  145,  but  the  following 
differences  are  to  be  noted:  The  medullary  groove,  Md,  is  deeper  and  more  trough- 
like  in  section  and  its  floor  merges  into  the  primitive  streak,  Pr.S,  or  axial  cord  of 
cells,  which  merges  below  with  the  entoderm  and  laterally  with  the  mesoderm.  The 
fusion  with  all  three  germ-layers  is  the  essential  characteristic  of  the  primitive 
streak.  The  mesoderm,  Mes,  of  the  present  section  is  voluminous,  and  shows  in 


EMBRYO  WITH  EIGHT  SEGMENTS. 


195 


he  embryo-region  no  trace  of  a  coelomatic  fissure.  Over  the  area  pellucida  there 
is  no  angioblast  between  the  mesoderm  and  entoderm,  it  not  having  yet  penetrated 
from  the  area  opaca. 

Figure    147    represents,    under   a   considerably   higher    magnification,    three    sec- 


Seg 


EC. 


mes 


En 


FIG.  147. — CHICK  EMBRYO  WITH  SEVEN  SEGMENTS.     TRANSVERSE  SERIES  510,  SECTIONS  311,  212,  172. 
Three  transverse  sections  across  the  caudal  end  of  the  medullary  groove.     A,  Section  through  one  of  the  segments. 
B,  Section  posterior  to  the  segments.     C,  Section  just  in  front  of  the  primitive  streak.     Md.gr,  Medullary 
groove,     nch,  Notochord.     EC,  Ectoderm,     mes,  Mesoderm.     En,  Entoderm.      X  230  diams. 


196 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


tions  at  different  levels  through  the  open  groove  of  a  slightly  younger  chick.  In  the 
first,  A,  the  groove  is  quite  deep  and  the  young  primitive  segment  is  shown.  At 
the  edge  of  the  groove  its  thick  walls  pass  over  continuously,  but  quite  abruptly, 
into  the  general  ectoderm,  EC,  covering  the  embryo.  Close  under  the  median  line 
of  the  medullary  groove  appears  an  oval  section  of  the  notochord,  nch.  The  ento- 
derm,  En,  is  quite  thin  and  somewhat  irregular,  as  is  shown  in  all  of  the  sections. 
In  B  the  medullary  groove  is  wide  open  and  quite  shallow,  the  notochord  is  much 
larger  and  extends  from  the  floor  of  the  medullary ,  groove  to  the  entoderm  and 
occupies  in  part  a  deep  notch  in  the  medullary  wall.  The  notochord  prevents  the 


Md.gr. 


FIG.  148. — THREE  SECTIONS  OF  A  CHICK  EMBRYO  WITH  EIGHT  SEGMENTS.     SERIES  642. 

A.  Section  366,  through  the  cephalic  end  of  the  primitive  groove.     B.  Section  400,  through  the  middle  of  the 
primitive  groove.     C.  Section  424,  through  the  caudal  end  of  the  primitive  groove. 

extension  of  the  mesoderm  across  the  median  line.  In  C  the  medullary  groove  is 
fading  out  and  merging  into  the  beginning  of  the  primitive  streak,  which  forms 
a  large  mass  of  cells  in  the  median  line  in  which  the  boundaries  between  the  germ- 
layers  cannot  be  determined.  Laterally  this  mass  of  tissue  passes  over  into  per- 
fectly distinct  germ-layers,  of  which  the  middle  or  mesoderm,  mes,  is  by  far  the 
most  voluminous.  The  walls  of  the  medullary  groove  are  crowded  with  nuclei 
which  lie  at  every  possible  level,  some  close  to  the  inner,  others  close  to  the  outer 
surface,  and  also  in  every  -possible  intervening  position.  The  nuclei  are  much 
crowded,  there  being  but  little  protopjasm.  No  distinct  cell  boundaries  can  be 
made  out.  The  nuclei  are  further  remarkable  on  account  of  their  very  conspicu- 
ous nucleoli. 


EMBRYO  WITH  TWENTY-FOUR  SEGMENTS. 


197 


Sections  through  the  Primitive  Groove  (Fig.  148). — In  all  of  these  we  find  merely  the 
three  germ-layers,  which  are  all  united  in  the  median  line  with  the  axial  band  of  cells, 
constituting  the  primitive  streak.  The  ectoderm  has  the  deep  furrow  which  toward 
the  head  runs  into  the  medullary  groove.  Caudad,  the  primitive  groove  widens 
out  and  is  gradually  lost.  The  thinning  out  of  the  mesoderm  should  also  be  no- 
ticed, as  the  series  is  followed  in  the  caudad  direction.  Under  a  higher  power  the 
character  and  arrangement  of  the  cells  comes  out  more  clearly  (Fig.  149). 


FIG.   149. — CHICK  EMBRYO  WITH  SEVEN  SEGMENTS.     TRANSVERSE  SECTION  ACROSS  THE  PRIMITIVE  GROOVE. 
EC,    Ectoderm,     mes.   Mesoderm.     Ent,   Entoderm.     Pr.g,   Primitive   groove.     The  large   black    dots  represent 

yolk-grains.      X  230  diams. 

Embryo  Chick  with  about  Twenty-four  Segments  and  Three  Gill-clefts   (about 

Forty-six  Hours'1  Incubation). 

The  following  description  will  apply  almost  equally  well  to  embryos  with  from 
twenty-six  to  twenty-nine  segments. 

Examination  in  toto. — The  specimen  as  a  whole,  as  in  the  fresh  state,  has  a 
grayish  tint  when  viewed  by  transmitted  light.  As  soon  as  it  is  hardened  the 
opacity  of  all  the  tissues  is  greatly  increased.  In  the  center  of  the  germinal  area 
is  the  very  conspicuous  area  pellucida,  which  is  somewhat  pear-shaped.  The  por- 
tion around  the  anterior  end  of  the  embryo  (Fig.  150,  A.p)  is  very  wide.  In  the 
center  of  the  area  vasculosa  appears  the  embryo,  the  head  end  of  which  is  twisted 
over  so  that  the  left  side  of  the  head  lies  against  the  yolk.  This  twisting  of  the 
neck-  and  head  so  that  they  become  asymmetrical  in  position  is  very  characteristic 
of  birds.  Below  the  .head  and  somewhat  to  the  right  may  be  seen  the  tubular 
heart,  Ht,  which,  in  the  fresh  specimen,  pulsates  regularly.  Around  the  area  pellu- 
cida comes  the  dark  area  opaca,  in  which  we  readily  distinguish  the  outer  boundary 
or  terminal  sinus  of  the  area  vasculosa.  In  this  there  is  already  a  well-developed 
network  of  blood-vessels  through  which  the  blood  is  circulating,  being  driven  by 
the  heart.  The  blood  moves  out  from  the  embryo  by  two  large  vessels,  A.vi, 
which  lie  symmetrically,  the  vitelline  or  omphalo-mesaraic  arteries.  These  arteries 
arise  from  the  dorsal  aorta  of  the  embryo  and  pass  out  to  the  area  vasculosa, 
over  which  they  ramify.  -The  blood  returns  to  the  heart  by  means  of  a  network 
of  small  vessels,  across  the  posterior  part  of  the  area  pellucida;  the  network  close 
to  the  embryo  fuses  into  two  larger  short  trunks,  one  each  side.  The  two  trunks 


198 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


are  the  anlages  of  the  omphalo-mesaraic  veins,  which  gradually  grow  out  and 
branch  in  the  extra-embryonic  region,  enlarging  at  the  same  time  (compare  Fig. 
53).  The  general  form  of  the  embryo  is  indicated  by  figure  i$£>.  In  the  region 
of  the  head  we  notice  the  very  well-marked  head-bend  which  is  established  in  the 
region  of  the  mid-brain,  M.b.  The  medullary  tube  in  the  region  of  the  head  is 


M.b. 


A.p. 


Am.f. 


A.o. 


bl.is. 


Seg.z.  J 


Aid. 


Pr.g. 


FIG.  150. — EMBRYO  CHICK  WITH  ABOUT  TWENTY-FOUR  SEGMENTS.     SURFACE  VIEW  FROM  THE  DORSAL  SIDE. 

A.c.Vf  Amnio-cardiac  vesicle.  Am.f,  Posterior  edge  of  the  amniotic  fold.  A.o,  Area  opaca.  A.p,  Area  pellucida. 
A.w,  Arteria  vitellina  (or  omphalo-mesaraica).  bl.is,  Blood-island  in  the  area  pellucida.  Ht,  Heart.  M.b, 
Mid-brain.  Md,  Medullary  canal  or  spinal  cord.  Op,  Optic  vesicle.  Ot,  Otocyst.  Pr.g,  Primitive  groove. 
Seg,  Primitive  segment.  Seg.z,  Segmental  zone,  i,  2,  Gill-arches.  X  15  diams. 

very  much  enlarged  and  is  divided  into  three  well-marked  primary  cerebral  vesicles, 
which  appear  distinctly  in  specimens  that  have  been  stained  and  cleared.  The 
first  of  these  is  quite  large,  and  at  its  side  lies  the  anlage  of  the  eye,  Op,  in  the 
center  of  which  one  readily  distinguishes  the  commencement  of  the  lens  as  a  smalt 
invagination  of  the  ectoderm  with  its  orifice  still  open.  The  second  cerebral  vesicle 
is  much  smaller  .than  the  first  in  every  dimension.  It  occupies  the  region  of  the 
head-bend  and  is  separated  from  the  first  vesicle  by  a  constriction,  and  from  the 


EMBRYO  WITH  TWENTY-EIGHT  SEGMENTS.  199 

thirds  vesicle  by  another  constriction.  The  third  vesicle  in  length  more  than  equals 
the  first  and  second  combined,  and  at  its  widest  part  is  nearly  equal  in  diameter 
to  the  second  vesicle.  It  tapers  out  toward  the  caudal  end  0f  the  embryo  and 
passes  over  into  the  much  smaller  portion  of  the  medullary  canal,  which  represents 
the  anlage  of  the  spinal  cord.  At  the  side  of  the  third  vesicle  we  can  see  an  open 
pit,  the  anlage  of  the  inner  ear  or  otocyst,  Ot.  On  the  side  of  the  neck  between 
the  third  cerebral  vesicle  and  the  heart  there  are  three  external  depressions  which 
bound  the  first  and  second  branchial  arches,  i,  2,  of  the  embryo.  Behind  each 
arch  the  depression  marks  the  site  of  a  gill-cleft.  The  first  is  the  longer,  the 
second  the  shorter.  Between  the  projecting  head  and  the  first  branchial  arch  the 
outline  of  the  embryo  makes  a  depression  which  marks  the  position  of  the  develop- 
ing, oral  cavity.  The  heart  is  a  large  tube,  Ht,  in  a  still  larger  pericardial  cavity 
(ccelom),  the  membranous  covering  of  which  is  somatopleuric.  The  omphalo- 
mesaraic  veins  join  the  venous  or  posterior  end  of  the  heart.  The  heart  is  very 
much  bent;  its  anterior  end  turns  toward  the  gill-clefts  and  there  gives  off  the 
primitive  aortic  branches,  which  finally  join '  again  so  as  to  form  the  median  dorsal 
aorta  which  sends  off  the  two  vitelline  arteries,  A.m.  On  either  side  of  the  med- 
ullary canal  can  be  seen  the  primitive  segments,  Seg.  The  first  of  these  which 
is  distinct  lies  close  behind  the  otocyst.  At  the.  posterior  end  of  the  embryo  addi- 
tional segments  are  still  forming,  and  the  precise  number  of  segments  varies  from 
embryo  to  embryo.  The  medullary  canal,  Md,  is  closed,  but  beyond  its  extreme 
limit  traces  of  the  primitive  groove,  Pr.g,  can  still  be  seen.  The  network  of 
blood-vessels  over  the  area  vasculosa  is  very  distinct  and  characteristic.  The  net- 
work, however,  does  not  yet  extend  into  the  body  of  the  embryo  proper.  The 
limit  of  the  body  of  the  embryo  is  suggested  by  the  darker  tissue,  Seg.z,  surround- 
ing the  spinal  cord,  Md,  on  either  side.  About  the  hinder  end  of  the  embryo,  both 
iri  the  pellucida  and  in  the  opaca,  appear,  a  number  of  small  spots,  the  blood- 
islands,  bl.is,  many  of  which  have  in  the  fresh  specimen  a  reddish  color.  In  hard- 
ened specimens  the  opacity  of  the  blood-islands  renders  them  conspicuous,  espe- 
cially in  the  area  pellucida. 

Embryo  Chick  with  Twenty-eight  Segments. 

The  Study  of  Transverse  Sections. — A  series  of  figures  from  transverse  sec- 
tions of  an  embryo  of  this  stage  is  herewith  presented.  They  have  been  selected 
so  as  to  show  the  principal  typical  structures.  The  position  of  the  sections  can 
be  followed  more  easily  by  comparing  each  transverse  section  with  figure  166, 
to  determine  its  place  and  the  organs  through  which  it  must  pass. 

Section  through  the  Right  Auditory  Imagination  (Fig.  151).  —  Owing  to  the 
curvature  of  the  neck-bend,  the  section  of  the  head  is  not  symmetrical.  It  passes 
through  both  the  hind-brain,  h.b,  and  the  fore-brain,  f.b.  Underneath  the  former 
appears-  a  small  structure,  nch,  the  notochord,  and  on  one  side  can  be  seen  the 
auditory  invagination,  O/,  which  is  formed  wholly  by  the  locally  thickened  ectoderm, 


200 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


which  is  elsewhere  quite  thin.  The  ectoderm,  EC,  covering  the  dorsal  side  of  the 
hind-brain  is  very  thin,  but  the  portion  in  front  of  the  auditory  invagination  is 
somewhat  thicker.  The  ectoderm  of  the  invagination  is  very  much  thickened  and 
contains  numerous  somewhat  crowded  nuclei  at  all  levels.  These  nuclei  are  rounded 
in  form  and  have  one  or  two  very  distinct  nucleoli.  On  the  posterior  side  of  the 
otocyst  there  is  very  little  mesoderm;  on  the  anterior  side,  much  more.  Between  the 

•  Epen. 
/f  "  ^K  ^-"' 

h.b.  __    M  Ec 


Ot.d 


nch. 


Ao. 


EC 


FIG.  151.— SECTION  OF  CHICK  EMBRYO  WITH  ABOUT 
TWENTY-EIGHT  SEGMENTS.  TRANSVERSE 
SERIES  92,  SECTION  73. 

Ao,  Aorta.  EC,  Ectoderm.  f.b,  Fore-brain,  h.b, 
Hind-brain.  mes,  Mesoderm.  nch,  Noto- 
chord. Ot,  Otocyst.  Vc,  Vein.  X  50  diams. 


FIG.  152. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT 
TWENTY-EIGHT  SEGMENTS.  TRANSVERSE 
SERIES  92,  SECTION  83. 

A o.i,  Prolongation  toward  the  fore-brain  of  the  first 
aortic  arch.  Ao.D,  Descending  aorta,  card, 
Anterior  cardinal  vein,  cl.pl,  Closing  plate. 
cl.i,  First  gill-pouch.  EC,  Ectoderm.  Epet. . 
Roof  of  hind- brain.  /.&,•  Fore-brain,  h.b, 
Hind-brain,  "mes,  Mesoderm.  nch,  Notochord 
Op,  Optic  vesicle.  Ot.d,  Right  otocyst.  Ot.s, 
Left  otocyst.  Ph,  Pharynx.  X  50  diams. 


developing  otocyst  and  the  notochord  there  is  a  blood-vessel,  Ve,  with  merely  endo- 
thelial  walls,  a  branch  of  the  cardinal  vein.  Between  the  hind-brain  and  fore- 
brain  near  the  notochord,  the  two  aortae,  Ao,  are  cut.  In  their  interior  there  can 
usually  be  seen  a  certain  number  of  nucleated  blood-cells  varying  somewhat  in 
size  and  appearance,  but  generally  having  a  rounded  form  with  distinct  outline  and 
a  well-defined  nucleolated  nucleus. 

Section  through  the  Left  Auditory  Invagination  (Fig.   152). — Owing  to  the  irregu- 
lar form  of  the  embryo  the  sections  through  the  otocyst  are  not  symmetrical.     The 


EMBRYO  WITH  TWENTY-EIGHT  SEGMENTS. 


201 


Epen. 


card. 


sent  section  shows  the  opening  of  the  left  otocyst,  Ot.s,  and  a  closed  section 
of  the  right  otocyst,  Ot.d.  At  its  lower  inner  edge  the  outer  boundary  of  the  wall 
of  the  otocyst  is  indistinct,  this  arppearance  being  due  to  the  union  of  the  cells  of 
the  acoustic  ganglion  with  the  wall  of  the  otocyst.  The  section  also  passes  through 
the  first  gill-cleft,  cl.i,  of  the  right  side,  and  shows  very  distinctly  indeed  the 
closing  plate,  cl.pl,  which  is  formed  by  a 
fusion  of  the  ectodermal  and  entodermal 
cells.  On  the  opposite  side  of  the  section 
the  same  cleft  is  imperfectly  shown.  On 
the  posterior  side  of  the  cleft  is  the  dorsal 
aorta,  Ao.D,  and  on  the  anterior  side  of 
the  cleft,  extending  toward  the  fore-brain, 
f.b,  appear  the  sections  of  the  two  pro- 
longations of  the  first  arches,  Ao.i,  toward 
the  fore-brain.  In  this  specimen  each  pro- 
longation forms  a  loop,  which  rejoins  its 
arch*  dorsally.  In  the  region  of  the  fore- 
'brain  appears  a  shaving  from  the  edge  of 
the  optic  evagination,  Op.  The  anterior 
cardinal  veins,  card,  appear  just  inside  of 
the  otocyst  close  to  the  ventral  wall  of  the 
hind-brain,  h.b. 

Section  through   the   Invagination   of  the 
Optic    Lens    (Fig.    153). — This    section    also 
passes    through    the    hind-brain,    h.b,    fore- 
brain,    f.b,     and     through     the     openings     of      FIG.  153. -SECTION  OE  CHICK  EMBRYO  WITH  ABOUT 
J  TWENTY-EIGHT      SEGMENTS.         TRANSVERSE 

both   invaginations   to   form  the  anlages,  L,  SERIES  92,  SECTION  96. 

of     the     lenses     Of    the  eye.      These    invagina-       Ao.D,  Descending  aorta.     Ao.i,  First  aortic  arch: 


Mdb. 


Ent. 


Ret. 


f.b. 


Ao.2,  Second  aortic  arch,  card,  Anterior  car- 
dinal vein.  cl.I,  First  gill-cleft.  cl.II,  Second 
entodermal  gill-cleft.  EC,  Ectoderm.  Ent, 
Entoderm.  Epen,  Roof  of  hind-brain,  f.b, 
Fore-brain,  h.b,  Hind-brain.  L,  Invagination 
of  lens.  Mdb,  Mandibular  arch,  mes,  Meso- 
defm:  .nch,  Notochord.  Op,  Optic  vesicle. 
Ph,  Pharynx.  Ret,  Retina.  X  50  diams. 


tions    bear   a  striking  resemblance  to  those 

which    form    the    otocysts.      The    ectoderm, 

EC,  over  the  roof  of  the  fore-brain  is  very 

thin  and  passes  abruptly  into  the  thickened 

layer  which  forms  the  wall  of  the  invagina- 

tion.     On  the   ventral  side  the  ectoderm  is 

somewhat  thicker.      The   wall   of  the  lentic 

vesicle  is  quite  thick;  its  nuclei  are  numerous,  but  are  situated  chiefly  on  the  meso- 

dermal  side  of  the  layer,  so  that  toward  its  outer  surface  the  layer  is  comparatively 

free  from  nuclei.     The   invagination  of  the  lens  rests  against  the  optic  vesicle,  the 

wall  of  which,  Ret,  next  to  the  lens  is  thicker  than  the  posterior  or  inner  wall  of  the 

optic  vesicle.     The  thickened  outer  portion  is  the  anlage  of  the  retina,  the  thinner  inner 

portion   is   the   anlage    of   the    pigment   layer   covering   the    retina.     The    fore-brain, 

f.b,  has  an  elongated  form  with  quite    thick    walls    crowded  with  nuclo'       Between 


202 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


Ao.D. 


cl.II. 


Ao.2. 


card. 


nch. 


cl.II. 


Ph.* 


Ht. 


EC. 


it  and  the  hind-brain  appears  the  cavity  of  the  pharynx,  Ph,  which  on  the  left 
side  of  the  embryo  shows  a  prolongation,  cl.I,  which  extends  almost  to  the  sur- 
face of  the  embryo.  This  prolongation  is  the  first  gill-pouch.  On  the  dorsal  side 
of  the  pharynx  appear  the  two  large  aortic  trunks,  Ao.D,  and  on  its  ventral  side 
the  two  smaller  first  aortic  arches,  Ao.i.  These  are  situated  in  the  mandibular 

branchial  arch,  Mdb,  which  is  well  marked 
externally  by  a  rounded  protuberance.  The 
distal  end  of  the  second  gill-pouch  is  shown 
on  the  right  side  of  the  section,  d,  II. 

Section  through  the  Optic  Stalks  (Fig.  154). 
—The  head  of  the  embryo  now  appears  quite 
isolated  from  the  body.  It  is  bounded  by  a 
distinct  layer  of  ectoderm,  EC,  and  contains 
the  very  large  fore-brain,  f.b,  which  gives  off 
on  either^  side  an  optic  evagination,  Op,  the 
walls  of  which  are  quite  thick,  about  the  same 
as  those  of  the  fore-brain  proper.  Each  optic 
evagination  is  widest  toward  the  side  of  the 
head  and  is  constricted  toward  the  brain, 
with  which,  therefore,  it  is  connected  by  a 
stalk  in  which  we  can  already  recognize  the 
anlage  of  the  optic  nerve.  Between  the  two 
optic  stalks  on  the  side  toward  the  pharynx 

„  the  floor  of  the  fore-brain  bends  downward  and 

FIG.   154. — SECTION  OF  CHICK  EMBRYO  WITH 

ABOUT  TWENTY-EIGHT  SEGMENTS.    TRANS-     almost   joins  the  superficial  ectoderm.       All  of 

VERSE  SERIES  92,  SECTION  104.  the  space  between  the  walls  of  the   fore-brain 

MTrunkoftheaorta.  AO.D,  Descending  aorta.     and    the        t;c   evagination   on  the.  one ,  hand, 

A  0.2,   Second  aortic  arch,     card,   Anterior  .  . 

cardinal    vein.     cl.II,    Second    entodermal      and    of    the     Superficial    ectoderm    of    the    head 

Fore-  on  the  other,  is  filled  with  undifferentiated 
mesenchyma.  In  this  tissue  blood-vessels, 
nerves,  lymphatics,  and  muscles  will  grow,  and. 
the  tissue  itself  is  to  produce,  the  cutis,  the 
subcutaneous  tissue,  the  skull,  the  dura  mater, 

arachnoid  membrane,  and  pia  mater.  We  have  in  the  present  undifferentiated 
stage  of  this  mesenchyma  a  most  striking  contrast  with  the  complicated  histo- 
logical  conditions  of  the  adult.  The  opposite  part  of  the  embryo  represents  .the 
cervical  region.  At  one  side -we  see  a  small  piece  of  the  heart  appearing,  Ht,  and 
higher  up  is  the  wide  pharynx,  Ph,  underneath  which  is  a  blood-vessel,  Ao,  the 
main  aorta.  To  the  left  appears  another  blood-vessel,  Ao.2,  a  portion  of  the 
second  aortic  arch.  The  pharynx  shows  on  one  side  the  prolongation  of  its  cavity 
which  constitutes  the  second  gill-pouch,  cl.II.  On  the  dorsal  side  of  the  pharynx 
'••^ending  aortas,  Ao.D,  that  on  the  right  of  the  figure  being  joined 


f.b. 


gill-pouch.  EC,  Ectoderm,  f.b, 
brain,  h.b,  Hind- brain.  Ht,  Heart,  mes, 
Mesoderm.  My,  Muscle  plate,  nch,  Noto- 
chord.  Op,  Optic  vesicle.  Ph,  Pharynx. 
X  50  diams. 


EMBRYO  WITH  TWENTY -EIGHT  SEGMENTS. 


203 


by  the  second  aortic  arch,  near  which  appears  an  accumulation  of  more  deeply 
colored  cells,  cl.II,  part  of  the  entodermal  wall  of  the  second  gill-pouch.  Between 
the  pharynx  and  the  hind-brain  we  have  a  round  section  of  the  small  notochord 
which  appears  quite  deeply  stained,  and  therefore  stands  out  conspicuously  from 
the  very  loose  mesenchyma  by  which  it  is  surrounded.  It  is  not  until  later  stages 
that  the  mesenchymal  cells  begin  to  crowd  around  the  notochord  to  constitute  the 
anlage  of  the  future  vertebral  column.  At  the  present  stage  the  differentiation  of 
the  axial  skeleton  around  the  notochord  has  not  begun.  As.  regards  the  hind-brain, 
h.b,  we  observe  that  its  sides  are  already  considerably  thickened,  but  its  dorsal 
wall  is  quite  thin  and  has  already  expanded  considerably,  thus  initiating  the 
formation  of  the  thin  ependymal  roof  of  the  fourth  ventricle.  On  either  side  of  the 
hind-brain  appears  a  blood-vessel,  card,  the  anterior  cardinal,  which  by  transforma- 
tion and  migration  is  to  lead  to  the  formation  of  the  jugular  veins  of  the  adult. 

Section  through  the  Aortic  End  of  the  Heart  (Fig.  155). — The  cervical  region 
of  the  head  and  the  tip  end  of  the  region  of  the  fore-brain  are  cut  separately. 
On  the  lower  side  of  the  pharynx  is  attached  the  double  heart-tube,  of  which  the 
endothelial  portion,  endo,  is  in  actual  contact  with  the  thick  entoderm,  En,  which 
forms  the  floor  of  the  pharynx.  The  heart-tube  shows  its  bend  toward  the  right 
of  the  embryo.  There  is  a  considerable  space  between  the  endothelial  heart  and 
the  muscular  heart,  m.ht,  and  this  space  is  almost  wholly  free  of  tissue,  except  in 
the  immediate  neighborhood  of  the  pharynx  itself.  Close  to  the  connection  of  the 
heart-tube  with  the  pharyngeal  floor  there  runs  off  on  either  side  the  membrane 
of  the  amnion.  Where  it  starts  from  the  embryo  the  amnion  has  considerable 
thickness  and  appears  somewhat  folded  in  the  section;  but  as  it  turns  to  cover 
the  embryo  it  becomes  very  thin.  It  consists  only  of  two  very  delicate  layers,  meso- 
dermic  and  entodermic,  both  one  cell  thick.  The  two  layers  lie  close  together,  but 
are  easily  distinguished.  On  the  right-hand  side  of  the  embryo  the  raphe  of  the 
amnion  may  be  observed,  raph,  and  in  this  section  it  is  constituted  by  only  two 
strands  of  mesoderm  which  pass  over  from  the  amnion  on  to  the  chorion,  Cho, 
or  membrana  serosa,  as  it  has  been  called  by  many  embryologists.  The  arrange- 
ment of  the  envelopes  of  the  head  is  somewhat  more  complicated.  Underneath  the 
left  side  of  the  section  of  the  cervical  portion  of  the  head  runs  the  splanchnopleure, 
Spl,  in  which  one  can  readily  distinguish  numerous  sections  of  blood-vessels,  which, 
on  the  side  toward  the  embryo,  are  covered  by  mesoderm,  and  on  the  side  away 
from  the  embryo  are  covered  by  entoderm.  If  we  follow  along  the  splanchnopleure 
to  a  point  near  the  section  of  the  region  of  the  fore-brain,  we  find  that  it  encounters 
a  circle  of  ectoderm,  EC,  which  surrounds  that  portion  of  the  head.  When  the 
splanchnopleure  reaches  this  ectoderm,  its  two  layers  divide  or  split  apart.  The 
mesoderm  bends  off  toward  the  right*  side  of  the  embryo  and  forms,  together  with 
a  portion  of  the  ectoderm,  a  part  of  the  true  amnion,  Am',  of  the  head.  The 


*  The  right    of   the    embryo, — the  left-hand  side  of  the  figure. 


204 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


entoderm,  Ent,  on  the  contrary  continues  in  the  same  direction  as  before,  until  it 
joins  the  ectoderm  on  the  left  side  of  the  head  to  form  the  pro-amnion,  Pro.am. 
Beyond  the  head  the  entoderm  and  mesoderm  again  unite  and  we  have  a  continua- 
tion of  the  splanchnopleure,  Spl.  Owing  to  the  development  of  the  pro-amnion, 
the  relations  of  the  fetal  envelopes  surrounding  the  head  are  complicated.  The 


d.III 


FIG.  155. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.     TRANSVERSE  SERIES  92, 

SECTION  114. 

Am,  Am',  Amnion.  Ao.D,  Descending  aorta.  Ao.2,  Second  aortic  arch  of  the  left  side.  Cho,  Chorion.  cl.II, 
Second  entodermal  gill-pouch.  cl.III,  Third  entodermal  gill-pouch.  EC,  Ectoderm.  En,  Entoderm  of 
pharynx,  endo,  Endothelial  heart.  Ent,  Entoderm  of  pro-amnion.  f.b,  Fore-brain.  Md,  Medulla  oblongata. 
mes,  Mesoderm  of  amnion.  m.ht,  Muscular  heart,  nch,  Notochord.  Pro.am,  Pro-amnion.  raph,  Raphe  of 
amnion.  Seg,  Segment.  Spl,  Splanchnopleure.  Ve,  Anterior  cardinal  vein.  X  50  diams. 

student  may,  however,  easily  satisfy  himself  that  the  layer,  EC,  in  figure  157,  is 
really  ectoderm  by  following  it  through  in  the  series  of  sections,  for  he  will  then 
find  that  it  becomes  continuous  in  other  regions,  on  the  one  hand,  with  the  ecto- 
derm of  the  true  amnion,  and,  on  the  other,  with  the  epidermis  of  the  body  proper. 
In  the  cervical  region  we  have  a  transverse  section  of  the  lower  portion  of  the 


EMBRYO  WITH  TWENTY-EIGHT  SEGMENTS.  205 

hind-brain,  Md,  corresponding  to  the  part  of  the  future  medulla  oblongata  near 
its  junction  to  the  spinal  cord.  Underneath  it  is  the  section  of  the  notochord,  nch, 
and  on  either  side  sections  of  a  secondary  somite,  Seg.  Just  below  each  somite 
is  a'  cardinal  vein,  Ve,  and  below  the  vein,  but  nearer  to  the  median  line,  lies 
the  dorsal  aorta,  Ao.D.  The  pharynx  expands  on  each  side;  the  prolongation  on 
the  left  of  the  embryo  is  the  second  gill-pouch,  cl.II,  that  on  the  right  is  the 
third  gill-pouch,  cl.III.  The  pharynx  itself  is  lined  by  entoderm,  En,  which  is 
very  thin  in  the  median  dorsal  line,  but  immediately  below  the  dorsal  aortae  it 
thickens  abruptly  and  continues  as  a  quite  thick  layer  on  to  the  ventral  side.  In 
the  median  ventral  line  it  forms  a  deep  groove,  and  in  the  walls  of  this  groove  we 
find  that  the  nuclei  are.  not  distributed  through  the  whole  thickness  of  the  ento- 
derm, but  occupy  chiefly  its  outer  or  basal  portions,  so  that  the  portion  of  the 
layer  next  the  cavity  of  the  groove  is  formed  almost  wholly  of  protoplasm.  At  the 
tip  of  the  gill-pouch  the  entoderm  has  come  into  actual  contact  with  the  ecto- 
derm, and  the  cells  of  the  two  germ-layers  have  there  united,  without  distin- 
guishable boundary  being  kept  between  the  layers.  The  fused  ectoderm  and  ent 
derm  constitute  the  closing  plate  of  the  gill-cleft,  and  such  a  plate  is  formed  at  the 
tip  of  every  gill-pouch.  On  the  left  side  of  the  ventral  surface  of  the  pharyn/ 
pears  the  section  of  the  second  aortic  arch,  A 0.2.  Opposite  but  higher  up  is 
the  section  of  the  right  third  aortic  arch.  By  following  along  through  a  few  sec- 
tions (in  the  series  here  studied,  from  four  to  six)  the  junction  of  these  arches 
with  the  endothelial  tube  of  the  heart  may  be  observed.  The  student  should  verify 
this  connection  and  satisfy  himself  that  the  endothelium  of  the  blood-vessels  is  a 
continuation  of  the  endothelium  of  the  heart.  This  fact  is  of  great  morphological 
and  physiological  importance.  Of  the  section  of  the  region  of  the  fore-brain  little 
need  be  said.  The  ectoderm  has  begun  to  thicken  somewhat.  •  The  walls  of  the 
fore-brain,  f.b,  itself  have  not  begun  to  show  any  differentiation  into  layers.  There 
is  a  considerable  development  of  mesenchyma  between  the  brain  and  the  superficial 
ectoderm.  » 

Section  through  the  Venous  End  of  the  Heart  (Fig.  156). — We  have  now  passed 
in  our  series  beyond  the  level  of  the  head,  so  that  no  part  of  that  is  included  in 
the  section.  The  general  topography  of  the  part  is  similar  to  that  of  the  preced- 
ing section  (Fig.  155),  but  there  are  many  important  differences  of  detail.  We 
are  now  in  the  region  of  the  spinal  cord,  proper,  Sp.c,  which  here  offers  to  us  its 
characteristic  early  embryonic  form.  It  is  oval  in  section,  -its.  walls  are  thickened 
on  each  side,  but  are  thinned  on  the  dorsal  side,  where  they  constitute  the  deck- 
plate,  and  on  the  ventral  side,  where  they  form  the  floor-plate;  the  cavity  is  narrow 
and  slit-like.  The  nofochord  '  close  under  the  ventral  side  of  the  medullary 
tube  and  below  it  i^  tbr  median  dorsal  aorta,  Ao,  a  single  and  very  large  vessel, 
which  is  formed  by  fi  <•  union  of  .v  the  two  dorsal  aortae  shown  in  figure  157, 
Ao.D.  Immediately  below  in*,  aorta  f,\!o,vs  the  pharynx,  Ph,  which  is  nov  more 
rounded  in  form  and  does  not  e.-iend  «ar  laterally.  Its  entodermal  lining  is  mod- 


206 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


erately  thick,  but  it  is  somewhat  thinner  near  the  median  dorsal  line.  On  either 
side  of  the  pharynx  the  mesodermal  layer,  mes,  is  very  thick  and  stands  out  con- 
spicuously, owing  to  its  dark  staining.  Above  the  pharynx  it  thins  out  and  passes 
over  on  to  the  somatopleure,',  Som,  and  so  on  to  the  amnion,  Am.  On  the  ventral 
side  of  jthe  pharynx  the  mesodermal  layer  passes  over  into  the  muscular  wall  of 


m.ht. 


FIG.  156. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.     TRANSVERSE  SERIES  9?, 

SECTION   144. 

Am,  Am',  Amnion.  Ao,  Aorta.  Au,  Cardiac  auricle.  Cho,  Chorion.  Coe,  Ccelom,  D.C,  Duct  of  Cuyier. 
EC,  Ectoderm.  Endo,  Endothelial  heart,  mes,  Mesoderm.  m.ht,  Muscular  heart.  My',  Primitive  segment. 
Ph,  Pharynx.  Raph,  Raphe  of  amnion.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  Spl,  Splanchnopleure. 
Ven,  Ventricle  of  heart.  X  50  diams. 

the  heart,  m.ht.  The  heart  itself  is  very  large;  it  has  two  tubes,  the  endothelial, 
endo,  and  the  muscular,  m.ht,  which  are  very  distinct.  The  endothelial  cavity  is 
very  large.  It  is  especially  expanded  immediately  underneath  the  pharynx  to  form 
the  auricular  end  of  the  heart,  which  receives  the  veins.  Throughout  a  large 
part  of  the  auricle  the  cndothelium  is  closely  fitted  against  the  muscular  wall. 
Farthei  ventralward  me  double  heart-tube  bends  tr  the  right  of  the  embryo  to 
form  the  ventricular  limb,  Ven,  in  whiph  the  epithelial  cavity  is  also  enlarged. 


EMBRYO  WITH  TWENTY -EIGHT  SEGMENTS.  207 

The  heart  as  a  whole  occupies  about  one-half  the  area  of  the  entire  section  of  the 
embryo,  being  of  relatively  enormous  proportions.  The  cardinal  veins,  D.C,  have 
moved  down,  as  compared  with  the  previous  section,  and  -are  now  found  to  lie  in 
the  somatopleure,  in  which  there  also  appear  several  sections  of  smaller  blood 
spaces  above  the  main  cardinal  vessel.  The  path  of  the  cardinal  through  the 
somatopleure  carries  it  toward  the  heart.  The  vertical  part  of  the  vessel,  which 
effects  a  union  with  the  heart,  is  known  as  the  common  cardinal.  The  common 
cardinals  also  deliver  the  blood  from  the  posterior  cardinals  to  the  heart.  They  are 
at  somewhat  different  levels  on  the  two  sides  of  the  embryo,  that  on  the  right 
side  being  lower  and  occupying  a  sort  of  prominence  on  the  mesothelial  side  of 
the  somatopleure.  If  the  cardinal  veins  are  traced  along  through  successive  sec- 
tions, it  will 'be  found  that  they  open  directly  into  the  auricles  of  the  heart,  cross- 
ing over  the  ccelom,  Cce.  The  crossing  is  accomplished  by  a  growth  of  the  somat- 
opleure which  unites  with  ther  wall  of  the  heart.  The  openings  of  these  veins  are 
at  this  stage  morphologically  symmetrical  and  are  entirely  distinct  from  the  open- 
ings of  the  omphalo-mesaraic  veins,  which  enter  the  heart  farther  tailward.  If 
sections  in  the  series  between  the  present  one  and  that  through  the  aortic  end  of 
the  heart  (Fig.  155)  be  examined,  it  will  be  found  that  the  heart  in -the  middle 
part  of  its  course  is  entirely  detached  from  the  pharynx,  so  that  the  heart-tube  is 
suspended  by  its  two  ends  from  the  ventral  side  of  the  pharynx. .  By  the  crossing 
of  the  cardinal  veins  the  portion  of  the  coelom,  Cce,  on  either  side  of  the  pharynx  is 
shut  off  from  the  portion  of  the  coelom  around  the  heart.  At  the  raphe,  raph,  of 
the  amnion  the  ectoderm  of  the  amnion  joins  that  of  the  chorion,  Cho.  In  the 
portion  of  the  somatopleure,  Am',  which  runs  from  the  raphe  to  the  embryo  there 
area  number  of  spaces  of  rounded  form  which  appear  like  so  many  vesicle?.  The 
nature  of  these  vesicles  is  uncertain.* 

The  secondary  somites,  My,  are  very  characteristic,  and  should"  be  studied 
with  a  higher  power.  The  somite  consists  of  an  outer  layer  and  an  inner  1,: 
"of  about  equal  thickness,  and  these  two  layers  pass  over  into  one  another  at  the 
dorsal  and  ventral  edges  of  the  segment.  They  are  closely  pressed  against  one 
another,  so  that  there  is  no  space  between  them.  The  outer  layer  is  more  deeply 
stained  than  the  inner;  its  nuclei  are  somewhat  less  distinct  and  are  rounded  in 
form.  Those  of  the  inner  layer  are  elongated  in  form,  as  may  be  easily  observed 
by  raising  and  lowering  the  focus.  The  outer  layer  is  quite  close  to  the  ectoderm, 
and  the  inner  layer  rests  against  the  large  mass  of  mesenchymal  tissue  which  sur- 
rounds the  spinal  cord,  notochord,  and  aorta. 

Section  through  the   Anlage  of  the  Liver   (Fig.    157). — In  this  section  the   general 
topography  is  similar  to  that  of  the  last,   so  that   we   need  describe  only  the  new 


*  They  seem  to  be  bounded  on  one  side  by  ectoderm,  on  the  other  by  mi-,oderm;  but  as  » 
the  significance  of  tht;se  vesicles,  I  cannot  express  any  opinion.     The  separaii-  <  •'^mn.sj?^' 
front  of,  and  independently  of,  the  omphalo-mesaraic  veins,  so  far  as  I 
It  is  a  con-  ; ological  importance. 


I 


208 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


structures  and  relations  which  appear.  A  little  piece  of  the  ventricular  limb  of 
the  heart  with  its  double  walls,  m.ht,endo,  still  appears.  The  section  is,  strictly 
speaking,  beyond  the  venous  end  of  the  heart  and  passes  through  the  sinus  venosus 
Si.  V,  which  is  formed  by  the  union  of  the  omphalo-mesaraic  veins  entering  the 
body  of  the  embryo  from  the  splanchnopleure  of  the  yolk-sac,  or,  in  other  words, 


endo. 


Ent. 


FIG.  157. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.     TRANSVERSE  SERIES  92, 

SECTION  165. 

Am,  Amnion.  Ao,  Aorta,  card,  Cardinal  vein.  Cho,  Chorion.  Cos,  Cce',  Ccelom.  EC,  Ectoderm,  endo 
Endothelial  heart.  Ent,  Entoderm.  Li,  Liver,  mes,  Mesoderm.  m.ht,  Muscular  heart. '  msth,  Meso- 
thelium.  My,  Primitive  segment,  nch,  Notochord.  raph,  Raphe  of  amnion.  Si.  V,  Sinus  venosus  of 
heart.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  Spl,  Splanchnopleure.  Ve,  Vein,  x,  Accumulation  of 
mesodermic  tissue  about  the  omphalo-mesaraic  vein.  X  50  diams. 

from    the   area    vasculosa.     ]n    the    splanchnopleure,   Spl,    there   is    a    thickening,   x, 
^derm  which    .narks   the  crossing  of  the  veins  from  the  yolk-sac  to '  the 
The   entoderm  of  the   embryo   forms   a  tube,  Ent,   which 
•so-ventral    diameter.     The    entoderm    itself    is    quite 
From  the   ventral  side  of  >he^  entoder- 


• 


EMBRYO  WITH   TWENTY -EIGHT  SEGMENTS. 


209 


mal  canal  spring  two  small  pouches  or  diverticula,  the  anlages  of  the  liver.  The 
left  diverticulum  is  well  shown  in  the  figure;  the  right  diverticulum  appears  a 
few  sections  farther  on.  It  is  especially  important  to  note  that  the  entodermal 
epithelium  of  the  hepatic  diverticulum  comes  into  immediate  contact  with  the  endo- . 
thelium  of  the  blood  spaces.  During  the  later  development  this  relation  is  preserved, 
and  there  is  a  complicated  intercrescence  of  the  entodermal  cells  constituting  the 
liver  and  of  the  vascular  endothelium.*  The  intercrescence  leads  to  the  forma- 


Som. 


Spl. 


Om.S. 


Om. D. 


FIG.  158. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMKNTS.     TRANSVERSE  SERIES  92, 

SECTION  179. 

Am,  Amnion.  Ao,  Aorta,  card,  Cardinal  vein.  Cho,  Chorion.  Cos,  Coelom.  Ent,  Entoderm.  In,  Intestine. 
msth,  Mesothelium.  My,  Muscle  plate,  nch,  Notochord.  Om.D,  Right  omphalo-mesaraic  vein.  Om.S, 
Left  omphalo-mesaraic  vein.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  Spl,  Splanchnopleure.  X  50  diams. 

tion  of  the  sinusoids,  which  are  highly  characteristic  of  the  liver  and  which  give 
rise  to  the  so-called  capillaries  of  the  hepatic  lobules  of  the  adult  liver.  These 
"capillaries"  are,  however,  'always  true  sinusoids,  and  morphological!;*'  not  capil- 
laries at  all.  Owing  to  the  junction  of  the  veins  and  liver,  a  portion  of  the  body 
cavity,  Ccef,  at  the  side  of  the  pharynx  is  shut  off  from  direct  Connection  with 
the  pericardial  cavity.  The  ridge  of  tissue  dividing  the  two  cavitjes  from  one  an- 
other is  the  tjeptum  transvcrsum.  11"  the  ser  ns  be  followed  through  tail- 

*  A  few  sections  anterior  to  this  the  beginning  of  the  inten  i 


210 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


ward,  it  will  be  found  that  at  this  stage  further  back  the  septum  transversum  is 
formed  also  upon  the  right  side  of  the  body  of  the  embryo.  The  mesothelium 
between  the  upper  division  of  the  ccelom,  Cce,  and  the  sides  of  the  entodermal 
canal  is  very  much  thickened  and  deeply  stained.  On  either  side  of  the  very  large 
median  aorta,  Ao,  and  just  above  the  ccelom,  appear  the  right  and  left  posterior 
cardinal  veins,  card.  Concerning  the  fetal  envelopes  little  need  be  said,  except 
to  call  attention  to  .the  large  raphe,  raph,  of  the  amnion,  which  is  now  a  rather 
conspicuous  ectodermal  thickening  and  seems  to.  be  formed  rather  at  the  expense 
of.  the  ectoderm  of  the  amnion  than  of  that  of  the  chorion.  Such  an  ectodermal 
raphe  is  very  characteristic  of  birds;  it  has  in  the  chick  a  considerable  extent  and 
therefore  appears  in  many  successive  sections  of  the  series. 

Som.  EC.  msth.  card.  My.  nch.  Sp.c.  card.s.  b.w.  Am.  Cho. 


In.     Ao.     Ve.        mes.     Ent. 

FIG.   159. — SECTION  OF  A  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.     TRANSVERSE  SERIES  92, 

SECTION  220. 
. 
Am,  Amnion.     .  1<-  JX,     jody-wall.     card,  Right  posterior  cardinal  vein,     card.s,  Left  cardinal    vein. 

i.     EC,  Ectoderm.     Ent,  Entoderm.     In,  Intestine,     mes,  Splanchnic  mesoderm.     msth,  Meso- 
i.     My,  Myotome.     nch,  Notochord.    Som,  Somatopleure.     Sp.c,  Spinal  cord.     Spl,  Splanchnopleure. 
Ve,  Vein.      X  50  diams. 

Section  through  the  Omphalo-mesaraic  Veins  (Fig.  158). — This  section  is  inter- 
mediate in  structure  between  figure  157  and  figure  159,  here  described.  We  are 
now  beyond  the  region  of  the  heart  and  liver.  The  cavity  of  the  intestine  is  open 
on  the  ventral  side,  so  thai  the  walls  of  the  intestine  pass  over  directly  into  the 
extra-embryonic  Splanchnopleure,  Spl,  in  which  are  lodged  the  verv^Vide  omphalo- 
mesaraic  veins,  Om.D  and  Om.S,  which  are  entering  the  body  of  the  embryo 
to  run  forward  past  the  liver  anlage  (Fig.  157)  to  join  the  posterior  or  venous  end 
of  the  heart.  It  will  also  be  noticed  that  the  amniotic  fold  does  not  join  its  fellow, 
and  therefore  has  no  raphe,  In  this  condition  the  amnion  is  said  to  be  "open." 

Section  through   the   Anterior  Portion   of  the   Open   Intestine    (Fig.    I5o/. — In   this 
section   the   intestinal   cavity  ,  7«,   being   without   a   ventral  wail,   opens  directly 
the   general    emi^t- mal   cavity   under   the    germinal   area   and   above    the    yolk-mu^ 
(compare  the  diagrams  .Figs.    29  and  45).     The  median  plane  of  the  embryo  is  still 


EMBRYO  WITH  TWENTY -EIGHT  SEGMENTS. 


211 


inclined  to  the  left.  The  extra-embryonic  somatopleure,  Som,  rises  in  two  high 
folds,  one  on  each  side  of  the  embryo;  the  inner  portion  of  each  fold,  Am,  belongs 
to  the  amnion,  the  outer  portion,  Cho,  to  the  chorion.  The  splanchnopleure,  Spl, 
passes  without  demarcation  into  the  wall  of  the  intestinal  cavity,  In.  The  ento- 
derm, Ent,  of  the  extra-embryonic  splanchnopleure  is  very  thin,  but  where  it  passes 
into  the  embryonic  region  toward  the  median  line,  it  thickens  a  little.  The  splanch- 
nic mesoderm  is  a  thin  layer  of  mesothelium  which,  of  course,  bounds  the  ccelom 
everywhere  and  can  be  followed  continuously  over  on  to  the  somatopleure.  The 
splanchnic  mesenchyma  is  loose  in  texture  and  surrounds  the  large  blood-vessels. 
The  splanchnic  mesoderm  on  either  side  of  the  intestinal  groove  appears  quite 
dark,  owing  to  the  condensation  of  the  tissue.  Whether  this  condensation  is  devel- 
oped from  the  mesothelium  or  from  the  mesenchyma  it  is  very  difficult  to  say. 


Som. 


N.     Seg.    Sp.c.     W.D.     EC.     Mes. 


Cat. 


mes' . 


FIG.  160. — SECTION  OF  A  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  -SEGMENTS.  TRANSVERSE  SERIES  92. 

SECTION  356. 

Cue,  Coelom.  EC,  Ectoderm.  Ent,  Entoderm.  Mes,  Somatic  mesoderm.  mes',  Splanchnic  mesoderm.  A', 
Xephrotome.  nch,  Notochord.  Seg,  Segment.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  Spl,  Splanchno- 
pleure. Ve,  Bloocl- vessel.  W.D,  Wolffian  duct.  X  50  diams. 

The  somatopleure,  Som,  where  it  becomes  embryonic,  increases  greatly  in  thickness 
and  forms  an  arch,  b.w,  which  is  the  beginning  of  the  formation  of  the  ventral 
body-wall  of  the  chick.  The  form  of  the  arch  indicates  the  commencing  closure 
of  the  embryonic  somatopleure  on  the  ventral  side,  by  which  the  body  of  the  em- 
bryo will  ultimately  become  shut  off  from  the  underlying  layers  of  the  blastoderm. 
In  the  median  plane  of  the  embryo  we  find  the  spinal  cord,  cut  somewhat  obliquely, 
the  notochord,  nch,  and  the  very  large  section  of  the  aorta,  Ao.  The  great 
transverse  width  of  the  aorta  is  due  to  its  approaching  division  toward  the  caudal 
end  of  the  body  to  form  the  two  branches  which  run  out  to  the  area  vasculosa  and 
are  known  as  the  omphalo-mesaraic  or  vitelline  arteries.  Before  they  leave  the 
body  of  the  embryo  each  of  these  arteries  gives  off  a  branch  which  continues  in 
the  body  of  the  embryo  not  far  from  the  notochord  and  close  to  the  entoderm. 
These  branches  subsequently  become  the  allantoic  arteries.  On  either  side  of 
the  spinal  cord  lie  the  secondary  somites,  My.  A  short  distance  from  the  aorta. 


212 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


on  either  side  appear  sections  of  two  rather  small  blood-vessels,  the  cardinal  veins, 
car d.  Between  the  vein  on  each  side  and  the  aorta  there  is  a  little  accumulation 
of  denser  tissue.  If  a  series  of  sections  is  followed  through,  the  Wolffian  duct 
may  be  traced  into  this  condensed  tissue,  and  when  the  duct  is  differentiated,  it 
will  take  the  place  of  this  tissue  between  the  aorta  and  the  vein. 

Section  through  the  Middle  Portion  of  the  Open  Intestine  (Fig.  160). — Compari- 
son of  this  section  with  the  preceding  is  instructive  as  an  illustration  of  the  fact 
that  the  differentiation  of  structures  is  found  less  advanced  as  we  proceed  toward 
the  caudal  end  of  the  embryo.  In  the  present  section  the  amniotic  -folds  can 
hardly  be  said  to  have  appeared  at  all,  although  the  ccelom,  Cce,  is  very  wide  in- 
deed, and  there  is  little  differentiation  in  either  the  somatopleure,  Som,  or  splanch- 
nopleure,  Spl,  between  the  embryonic  and  extra-embryonic  regions.  The  ento- 


Som.     Cce.     Cce.' 


nch. 


In.     Ent. 


FIG.  161. — SECTION  OF  A  CHICK  EMBRYO  WITH  TWENTY-EIGHT  SEGMENTS.  TRANSVERSE  SERIES  92,  SECTION  419. 
Cce,  Ccelom.  Cce' ,  Diverticulum  of  the  ccelom.  Ent,  Entoderm.  In,  Intestinal  cavity,  mes,  Mesoderm.  nch, 

Notochord.    Som,  Somatopleure.    Sp.c,  Spinal  cord.    Spl,  Splanchnopleure.    S.z,  Segmental  zone.      X  50 

diams. 

derm  is  a  little  thicker  in  the  embryo  than  in  the  extra-embryonic  territory.  A 
similar  difference  may  be  observed  in  the  ectoderm.  The  embryonic  mesoderm  in 
both  somatopleure  and  splanchnopleure  is  considerably  more  developed '  and  much 
denser  than  in  the  extra-embryonic  parts.  The  axial  structures  of  the  embryo — 
namely,  the  spinal  cord,  Sp.c,  and  notochord,  nch — are  about  the  same  as  further 
forward,  but  the  mesoderm  is  much  less  advanced  than  further  headward  as  is 
evidenced  by  the  small  amount  of  mesenchyma  above  the  axial  structures  and 
by  the  slight  differentiation  of  the  mesothelium.  The  condition  of  the  segments 
and  their  relations  to  the  somatic  and  splanchnic  mesoderm  are  closely  similar  to 
those  represented  in  figure  46.  Each  somite  consists  of  a  larger  part,  Seg,  of 
rounded  outline,  close  to  the  medullary  tube,  and  of  a  narrower  part,  the  nephro- 
tome,  N,  which  connects  the  inner  portion  of  the  somite  with  the  lateral  meso- 
derm. The  secondary  somite  consists  of  a  distinctly  marked  wall  which  extends 
around  underneath  the  ectoderm  and  against  the  side  of  the  medullary  tube,  and 
of  a  thick  inferior  wall  which  fills  up  also  the  center  of  the  somite.  Between  the 
nephrotome  and  the  entoderm  are  small  blood-vessels,  Ve. 

Section    through    the    Posterior   Portion    of   the    Open     /;  Fig.    161). — This 


EMBRYO  WITH  TWENTY-EIGHT  SEGMENTS. 


213 


section  is  similar  to  the  last,  but  we  may  note  especially  the  following  differ- 
ences: The  spinal  cord,  Sp.c,  shows  a  comparatively  large  cavity,  which  is  widest 
on  the  dorsal  side,  so  as  to  be  somewhat  triangular  in  section.  In  place  of  the 
segments  we  have  only  the  mass  of  cells,  S.z,  which  constitutes  the  segmental  zone, 
out  of  which  later  segments  will  be  differentiated.  The  segmental  zone,  S.z,  is 
of  a  rather  loose  texture  and  merges  without  boundary  into  the  somewhat  denser 
mesenchyma  of  the  somatopleure  and  splanchnopleure  of  the  embryo.  The  dense 
tissue  of  the  somatopleure  extends  much  farther  laterally  than  the  corresponding 
tissue  in  the  splanchnopleure.  The  notochord,  nch,  is  very  large  and  fills  out  the 
entire  space  between  the  ventral  boundary  of  the  spinal  cord  and  the  entoderm, 
and  though  the  mesoderm  comes  in  contact  with  the  notochord,  it  does  not  sur- 
round it,  the  relations  here  representing  an  earlier  stage  of  development  than  any 


Som.     EC.  Cce.          cau.i.  Sp.c.  nch.     S.z. 


Mes. 


Ent. 


All. 


FIG.  162.— SECTION  OF  A  CHICK  EMBRYO  WITH  TWENTY-EIGHT  SEGMENTS.     TRANSVERSE  SERIES  92,  SECTION  424. 

All,  Allantois.  cau.i,  Caudal  intestine.  Cce,  Coelom.  EC,  Ectoderm.  Ent,  Entoderm.  Mes,  Mesoderm. 
mes' ' ,  Splanchnic  leaf  of  mesoderm.  nch,  Notochord.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  Spl,  Splanch- 
nopleure. S.z,  Segmental  zone  of  mesoderm.  Ve,  Blood-vessel.  X  50  diams. 

which  we  find  further  head  ward.  The  entoderm,  Ent,  of  the  embryonic  region  is 
considerably  thickened  and  forms  an  intestinal  channel,  In,  of  very  characteristic 
form;  for  the  top  of  this  channel  is  nearly  horizontal,  while  the  sides  are  vertical 
and  form  a  distinct  angle  with  the  top.  In  the  midst  of  the  mesoderm,  on  either 
side  of  the  intestine,  there  is  a  small  cavity,  Cce',  which  in  two  or  three  sections 
further  forward  is  found  to  unite  with  the  general  cavity  of  the  ccelom.  The 
morphological  meaning  of  this  special  pocket  of  the  body-cavity  is  unknown. 

From  this  point  onward  in  the  series  changes  in  the  appearance  of  the  -sec- 
tions take  place  very  rapidly.  The  two  sections  next  to  be  described  are  quite 
close  in  the  series  to  the  present  one. 

'Section  through  the  Caudal  Intestine  (Fig.  162). — In  this  section  we  encounter 
the  singular  fusion  of  the  germ-layers  which  is  characteristic  of  the  caudal  extremity 
of  all  vertebrate  embryos  during  early  stages.  In  the  median  line  we  see  three 
distinct  cavities.  The  dorsal  of  these  may  be  readily  identified  as  the  continuation 
of  the  cavity  of  the  spinal  cord.  The  middle  and  ventral  cavities  are  entodermal; 
the  upper  of  the  two  entodermal  cavities,  cau.i,  represents  a  prolongation  of 
the  entodermal  cavity  into  the  developing  tail  of  the  embryo  (compare  Fig.  16). 


214 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


The  lower  cavity  is  the  anlage  of  the  allantois,  All,  which  is  destined  to  grow  out 
during  the  next  few  days  into  a  relatively  large  round  vesicle.  The  tissue  on  the 
ventral  side  of  the  spinal^  cord,  Sp.c,  is  connected  by  a  band  of  cells  with  the 
wall  of  the  caudal  intestine,  cau.i.  If  the  sections  just  in  front  are  studied  care- 
•fully,  it  can  be  easily  observed  that  the  notochord  also  passes  over  without  boun- 
dary into  the  same  band  of  cells,  which  is  a  mass  representing  the  fusion  of  the 
walls  of  the  medullary  canal  of  the  intestine  and  of  the  tissue  of  the  notochord. 
In  this  fused  tissue  we  can,  with  our  present  means,  detect  no  signs  of  the  corning 
differentiation.  Just  as  the  walls  of  the  caudal  intestine  are  fused  with  the  tissues 
on  the  dorsal  side,  so  also  are  they  fused  on  the  ventral  side'  with  the  tissue  of 
the  allantois.  If  we  follow  the  tissues  laterally,  we  see  that  they  merge  into  the 
mesoderm  proper.  From  the  mesoderm  there  h^,s  been  a  distinct  upgrowth  of 


Cos.        EC.     Mes.          Sp.c.     nch.   S.z. 


Som. 


mes.'     Ent. 


All. 


Ve.     msth.     Spl. 


FIG.  163. — SECTION  OF  A  CHICK  EMBRYO  WITH  TWENTY-EIGHT  SEGMENTS.  TRANSVERSE  SERIES  92,  SECTION  427. 
All,  Allantois.  Cce,  Coelom.  EC,  Ectoderm.  Ent,  Entoderm.  Mes,  Mesoderm.  mes',  Splanchnic  mesoderm. 

msth,    Mesothelium.     nch,    }STotochord.     Som,    Somatopleure.     Sp.c,   Spinal    cord.     Spl,    Splanchnopleure. 

S.z,  Segmental  zone  of  mesoderm.     Ve,: Blood-vessel.      X  50  diams. 

tissue  of  rather  loose  texture  on  either  side  of  the  medullary  canal  to  form  the 
segmental  zone, 'S.z. 

Section  through  the  Allantois  behind  the  Intestine  (Fig.  163). — This  section  is 
onjy  three  in  the  series  beyond  that  last  described,  yet  it  is  posterior  to  the  caudal 
intestine  and  shows,  therefore,  more  completely  the  fusion  of  the  structures  in 
the  axial  region.  Except  for  the  absence  of  the  caudal  intestine,  the  description 
of  the  last  section  might  apply  also  to  this.  The  shape  of  the  spinal  cord,  Sp.c, 
is  somewhat  different,  and  its  merging  on  the  ventral  side  with  the  underlying 
tissues  is  more  marked.  The  cavity  of  the  allantois  is  smaller  and  almost  slit- 
like.  The  other  differences  do  not  call  for  special  description. 

Horizontal  Section  (Fig.  164). — The  student  will  find  it  profitable  to  make  a 
series  of  sections  in  the  horizontal  plane,  trying  to  cut  them  as  nearly  as  possible 
parallel  with  the  median  plane  of  the  fore-brain  and  mid-brain. 

The  accompanying  figure  164  is  from  a  section  of  such  a  series.  It  shows 
very  clearly  the  general  form  of  the  embryo,  the  curvature  of  the  neck,  the  sharp 
angle  of  the  head-bend,  and  the  almost  straight  body.  In  the  section  represented 
the  lung  stretch  of  the  cavity  of  the  fourth  ventricle  or  hind-brain,  Ven. IV.  \< 


EMBRYO  WITH  TWENTY-EIGHT  SEGMENTS. 


215 


Ven.IV 


nch' 


FIG.  164. — HORIZONTAL  SECTION  OF  A  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS. 
Am,  Am',  Am",  Amnion.  Ao,  Aorta.  C.ao,  Cardiac  aorta.  Cce,  Crelom.  D.Ao,  Dorsal  aorta.  Dieii,  Dien- 
cephalon.  Endo,  Endothelial  heart.  H,  Cerebral  hemisphere.  M.b,  Mid-brain.  Md,  Medullary  tube. 
Mdb,  Mandible,  m.ht,  Muscular  heart,  nch,  nch',  nch",  Notochord.  Op,  Optic  vesicle.  Ph,  Pharynx. 
Seg,  Segment.  Seg.z,  Segmental  zone  of  mesoderm.  Som,  Somatopleure.  Sp.c,  Spinal  conl.  Yen,  Yen- 
tricle.  Yen.  IV,  Fourth  ventricle  or  cavity  of  the  hind-brain.  X  30  diams. 


216  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

well  shown,  and  it  can  be  readily  seen  that  the  hind-brain  is  nearly  equal  in 
length  to  the  mid-  and  fore-brains  combined.  In  the  floor  of  the  hind-brain  ap- 
pears a  series  of  curved  notches  corresponding  to  the  neuromeres.  Only  a  shaving 
from  the  side  of  the  mid-brain,  M.b,  and  two  similar  shavings  from  the  two  parts 
of  the  fore-brain,  the  diencephalon,  Dien,  and  the  cerebral  hemispheres,  H,  appear 
in  the  section.  The  optic  nerve  is  cut  across  and  appears  as  a  hollow  tube. 
Underneath  the  hind-brain  a  piece  of  the  pharynx,  Ph,  is  cut,  and  below  the 
pharynx  is  the  large  projecting  heart,  which  is  very  clearly  shown  to  consist  of 
an  inner  or  endothelial  tube,  Endo,  and  an  outer  mesothelial  tube,  m.ht,  the  anlage 
of  the  muscular  portion  of  the  heart.  The  endothelial  tube  is  cut  twice;  the 
upper  portion,  Ao,  is  the  aortic  trunk,  the  lower  portion,  Ven,  corresponding  to 
the  ventricle.  The  heart  is,  as  it  were,  suspended  from  the  lower  wall  of  the 
pharynx.  The  entoderm  of  the  pharynx  is  very  thin  on  the  dorsal  side,  and 
thicker  on  the  ventral  side.  Between  the  head  and  the  pharynx  one  can  see  the 
projecting  mandibular  process,  Mdb.  The  small  space  to  the  right  of  this  process 
in  the  figure,  between  it  and  the  head,  corresponds  to  the  cavity  of  the  mouth. 
Close  to  the  mandibular  process,  on  the  side  toward  the  heart,  springs  the  amnion 
of  the  embryo,  Am,  which  passes  close  around  the  head  of  the  embryo  lying  very 
near  it,  and  can  be  followed  down  to  where  it  rejoins  the  posterior  end  of  the 
embryo,  on  the  left-hand  side  of  the  figure.  Underneath  the  posterior  part  of  the 
hind-brain  can  be  seen  a  small  piece  of  the  notochord,  nch.  The  notochord 
appears  twice  more  in  the  section,  nch'  and  nch" ,  in  the  dorsal  region  of  the  em- 
bryo. From  the  end  of  the  hind-brain  the  cervical  region  curves  to  the  right. 
In  it.  there  is  a  large  cavity,  D.Ao,  the  dorsal  aorta.  To  the  left  of  the  dorsal 
aorta  we  begin  to  get  the  primitive  segments,  which  are  very  distinctly  marked. 
They  become  gradually  wider  and  wider  as  we  proceed  toward  the  caudal  end 
of  the  embryo.  There  also  they  are  less  advanced  in  their  development.  A 
small  bit  of  the  spinal  cord  appears  in  section,  Md.  From  the  extreme  inferior 
end  of  the  section  a  prolongation  of  the  somatopleure  can  be  seen  which  also 
leads  off  into  the  formation  of  the  amnion,  Am".  There  appears  again  a  piece, 
Sp.c,  of  the  spinal  cord  and  a  fragment  of  the  notochord,  and  on  either  side  of 
this  a  segmental  zone,  Seg.z,  of  the  mesoderm.  On  the  right  there  shows  a  small 
portion  of  the  body-cavity,  Cos,  distinctly  bounded  on,  both  sides.  Its  exterior 
boundary  is  a  piece  of  the  true  body-wall,  Som,  of  the  embryo,  and  close  by  it  is 
another  portion  of  the  amnion,  Am'.  How  this  is  possible  may  be  readily  under- 
stood by  comparison  of  this  figure  with  figure  161,  which  represents  a  transverse 
section  of  a  similar  embryo  in  this  region. 

Histological  Differentiation  of  the  Chick  Embryo  with  Three  Gill-clefts. 

It  is  important  that  the  student  make  a  thorough  examination  and  study 
with  a  high  power  of  all  the  cells  and  tissues  of  the  embryo  at  this  stage  so  as  to 
familiarize  himself  with  the  embryonic  characteristics  of  the  germ-layers.  The 


HISTOLOGICAL  DIFFERENTIATION.  217 

cellular  homogeneity  of  the  embryo  is  strikingly  evidenced  by  the  nuclei,  which 
in  all  parts  of  the  embryo  are  very  similar  in  size,  shape,  and  structure.  They 
are  all  rounded  in  form,  varying  between  spherical  and  slightly  oval  outlines, 
which  are  seldom  quite  regular.  The  outline  of  the  nucleus  is  always  well  marked, 
there  being  a  supefficial  layer  of  nuclear  substance,  which  gives  a  darker  appear- 
ance to  the  edge  of  the  nucleus.  In  the  interior  there  is  a  single  or  sometimes 
two,  very  rarely  three,  nucleoli,  which  are  quite  large  and  stain  deeply.  The 
strands  of  substance  between  the  nucleolus  and  the  outer  part  of  the  nucleus  are 
very  slight,  and  the  space  around  the  nucleolus,  therefore,  appears  light.  The 
protoplasm  of  the  cells  is  never  large  in  amount,  so  that  the  cell-body  about  each 
nucleus  is  not  conspicuous,  except  in  the  case  of  the  blood-corpuscles,  which  are, 
in  this  respect,  somewhat  more  advanced  than  the  other  cells  of  the  embryo. 

The  ectoderm  offers  chiefly  variations  in  its  thickness,  being  very  much  at- 
tenuated in  some  parts,  as,  for  instance,  in  the  posterior  portion  of  the  head,  where 
the  outer  ectoderm  overlies  the  hind-brain.  Most  of  the  epidermal  parts  have  be- 
gun to  increase  in  thickness,  and  contain  nuclei  in  two  or  even,  three  layers. 
There  are  several  special  thickenings  of  the  epidermal  layer,  for  which  the  name 
of  plakodes  has  been  proposed  (compare  page  76).  At  the  present  stage  three 
pairs  of  plakodes  are  seen.  The  first  is  the  pair  of  areas  which  are  to  be  invagi- 
nated  to  form  the  olfactory  pits;  the  second  is  the  pair  which  are  already  invagi- 
nated  to  form  the  anlages  of  the  lenses  of  the  eyes,  and  the  third  pair  is  also 
invaginated  to  form  the  otocysts.  The  portion  of  the  ectoderm  which  forms  the 
medullary  tube  is  also  very  much  thickened,  except,  of  course,  so  far  as  the  floor- 
plate  and  deck-plate  have  been  differentiated.  In  both  the  plakodes  and  in  the 
thickened  portions  of  the  medullary  wall  the  nuclei  occupy  nearly  the  whole  thick- 
ness of  the  layer,  being  themselves  several  layers  deep.  They  are,  however,  par- 
tially absent  from  that  portion  of  the  ectoderm  which  is  near  the  original  external 
or  free  surface.  Close  to  this  surface  there  are,  however,  a  certain  number  of 
nuclei,  the  large  majority  of  which  are  in  various  phases  of  division,  as  shown  by 
the  numerous  mitotic  figures.  No  mitoses  appear,  except  in  tne  superficial  portion 
of  the  layer.  Over  the  greater  part  of  the  amnion  the  ectoderm  is  so  very  thin 
as  to  resemble  almost  an  adult  endothelium,  but  over  the  chorion  or  serous  mem- 
brane it  is  a  little  thicker. 

The  entoderm  appears  in  three  distinct  forms:  first,  the  large,  long,  columnar 
cells  of  the  area  opaca;  second,  the  very  thin  cells  of  the  area  pellucida;  and, 
third,  the  somewhat  thicker  cell-layer  in  the  embryo  proper.  For  an  account  of 
the  cells  of  the  area  opaca  and  area  pellucida  see  page  64.  The  entoderm  in  the 
embryo  presents  considerable  variations  in  thickness  which  have  been  pointed  out 
in  the  descriptions  of  the  sections.  Where  it  is  thick  enough  to  permit  it,  the 
nuclei  are  disposed  in  several  layers,  and  in  such  places  we  find  that  the  nuclear 
divisions  take  place  only  in  the  superficial  portion  of  the  entoderm,  the  phenome- 
non here  being  similar  to  that  which  we  have  already  noted  in  the  ectoderm.  The 


218  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

notochord  has  a  sharply  defined  outline,  as  if  bounded  by  a  distinct  membrane. 
It  contains  nuclei  which  are  quite  closely  placed,  but  it  does  not  show,  at  least 
in  ordinary  preparations,  any  recognizable  division  into  separate  cells. 

The  mesoderm  offers  several  varieties,  not  so  much  in  the  character  of  the 
single  cells  as  in  their  methods  of  grouping.  We  notice,  first,  that  there  are  parts 
of  the  mesoderm  which  are  quite  thick,  and  in  which  we  cannot  perceive  any 
division  into  mesothelium  and  mesenchyma.  Such  a  thick  layer  of  mesoderm  may 
be  observed  at  either  side  of  the  pharynx  (Figs.  156,  Ph,  and  157),  or,  again, 
toward  the  caudal  end  of  the  embryo  in  both  the  somatopleure  and  splanchnopleure, 
occupying  a  larger  territory  in  the  former  than  in  the  latter  (Fig.  163).  But  for 
the  most  part  the  mesoderm  has  progressed  beyond  this  stage  and  shows  clearly 
the  differentiation  of  a  thin  mesothelial  layer  lining  the  coelom  and  the  scattered 
mesenchymal  cells.  The  mesothelium  is  quite  thin  in  some  parts,  almost  or  quite 
as  thin  as  adult  endothelium.  The  mesenchyma  consists  of  cells  with  small  proto- 
plasmic bodies  connected  together  by  fine  threads  of  protoplasm  and  with  a  trans- 
parent homogeneous  matrix  between  the  cells.  It  varies  greatly  in  appearance  ac- 
cording as  the  cells  are  more  or  less  closely  crowded  together,  or  widely  separated 
from  one  another.  These  differences  we  designate  as  varying  degrees  of  condensa- 
tion in  the  mesenchyma.  The  variations  occur  in  a  perfectly  definite  and  constant 
manner,  though  we  are  far  from  understanding  yet  either  the  cause  or  the  morpho- 
logical significance  of  these  variations.  The  secondary  somites  vary  greatly  in 
structure,  because  they  are  in  unlike  stages  of-  differentiation,  those  toward  the  tail 
being  least,  and  those  in  the  cervical  region  most,  advanced.  We  can,  therefore,  in 
a  single  embryo  observe  several  phases  of  the  breaking-up  of  the  inner  wall  of 
the  somite  to  form  mesenchyma  about  the  medullary  tube  and  notochord.  The 
transformation  is  accomplished  by  a  spreading  out  and  moving  asunder  of  the 
cells,  and  we  can  also  trace  a  gradual  differentiation  of  the  muscle-plate,  out  of 
the  inner  portion  of  the  somite.  The  external  layer,  or  so-called  cutis-plate, 
offers  an  apparently  more  ,or  less  epithelioid  structure  in  all  of  the  somites.  The 
Wolffian  duct  is  differentiated  only  through  a  part  of  the  embryo.  It  is  a  small 
cord  of  cells  that  has  as  yet  no  central  cavity.  The  blood-vessels  are  formed  solely 
by  the  endothelium  (angioblast).  There  is  nowhere  any  condensation  of  the  mesen- 
chyma about  the  blood-vessels  as  yet.  There  are  no  capillaries  whatever  in  the 
embryo.  One  of  the  most  important  vascular  modifications  has,  however,  been 
initiated  in  the  anlage  of  the  liver,  where  we  find  the  vascular  endothelium  com- 
ing into  close  contact  with  the  entodermal  cells  of  the  liver,  preparatory  to  the 
later  complete  differentiation  of  the  hepatic  sinusoids.  The  blood-corpuscles  are 
round  in  form  with  fairly  distinct  outlines.  Their  protoplasmic  bodies  are  much 
larger  than  those  of  any  other  cells  of  the  embryo  at  this  stage,  but  their  nuclei 
resemble  in  size  and  structure  those  of  other  tissues. 


CHAPTER  VI. 
STUDY  OF  PIG  EMBRYOS. 

Method  of  Obtaining  Embryos. 

The  pig  is  recommended  for  embryological  study  because  specimens  of  the 
embryos  in  sufficiently  early  stages  can  be  obtained  at  the  larger  packing  estab- 
lishments in  considerable  numbers  and  with  little  trouble  or  expense.  When 
this  material  is  not  obtainable,  rabbit  embryos  may  be  substituted,  as  these  ani- 
mals are  easily  kept  and  breed  'freely  (compare  page  166).  The  enormous  pre- 
cocious development  of  the  chorionic  vesicle  in  pigs  produces  an  enlargement 
of  the  uterus  which  is  usually  sufficient,  by  the  time  the  embryo  has  attained  a 
length  of  6  mm.,  to  be  observable  to  the  untrained  eye.  It  is,  therefore,  only 
necessary  to  ask  the  man  who  removes  the  viscera  from  the  pigs  to  lay  aside 
for  examination  all  of  the  uteri  which  appear  distended.  These  should  not  be 
turned  about  violently,  but  handled  carefully  and  should  be  opened  immediately. 
As  soon  as  the  ovum  is  exposed  it  will  probably  be  ruptured,  and  there  will 
occur  a  free  outflow  of  opalescent  fluid,  amniotic  and  allantoic.  With  the  aid 
of  .fine  forceps  and  a  horn  spoon  the  embryo  may  be  lifted  up — and  it  should 
on  no  account  be  directly  .touched — and  transferred  to  a  dish  containing  Muller's 
fluid,  in  which  the  specimen  should  remain  for  five  or  ten  minutes.  It  is  then 
transferred  with  the  help  of  the  horn  spoon  to  Zenker's  fluid.  Metal  instruments 
cannot  be  used  on  account  of  the  corrosive  sublimate  in  the  Zenker  solution.  In 
one  or  two  hours  the  embryo  should  be  transferred  to  fresh  Zenker  solution  and 
left  therein  a  varying  length  of  time,  according  to  the  size  of  the  specimen.  In 
general  it  may  be  said  for — 

Pigs  of  6  to  9  mm 12  hours. 

Pigs  of  12  mm. ..:... -  .  24  hours. 

Pigs  of  15  mm 36  hours. 

Pigs  of  20  to  25  mm 48  hours. 

It  is  undesirable  to  leave  any  specimen  in  the  Zenker  solution  more  than 
forty-eight  hours.  The  Muller's  fluid  is  used  for  cleaning  the  specimen.  It  causes 
a  granular,  non-adherent  coagulum  to  form  from  the  fetal  fluids.  If  the  speci- 
men is  put  directly  into  Zenker's  fluid,  a  fibrous  coagulum  is  formed  which  often 
adheres  closely  to  the  embryo  so  as  to  obscure  its  shape.  Such  a  fibrous  co- 

219 


220  STUDY  OF  PIG  EMBRYOS. 

agulum  cannot  be  removed  without  injuring  the  embryo.  After  having  remained 
a  .proper  length  of  time  in  the  Zenker  solution,  specimens  are  further  washed 
for  twenty-four  hours  in  running  water,  and  then  treated  with  alcohol  and  iodine 
in  the  usual  manner. 

The  Making  of  Serial  Sections. 

Specimens  should  be  colored  with  alum  cochineal  in  toto,  then  imbedded  in 
paraffin  and  cut  into  serial  sections  according  to  the  directions  given  in  Chapter 
VIII.  It  is  advantageous  to  apply  a  counterstain — orange  G  is  recommended. 

Selection  of  the  Planes  of  Section  and  the  Stages  for  Practical  Study. 

It  is  customary  to  distinguish  three  fundamental  planes — the  transverse,  the 
sagittal,  and  the  frontal.  It  is  impossible  to  so  define  these  planes  that  the  defini- 
tion shall  be  exact  for  all  stages.  But  in  general  it  may  be  said,  reference 
being  had  to  the  entire  embryo,  that  the  transverse  plane  is  one  which  will  be  at 
right  angles  to  the  notochord  and  medullary  tube  at  the  level  of  the  heart;  that 
the  frontal  plane  will  be  one.  at  right  angles  to  this,  passing  symmetrically  through 
the  limbs  of  the  embryo;  and,  finally,  that  the  sagittal  plane  is  one  parallel  to 
the  median  plane  of  the  body.  As  in  younger  embryos  the  form  is  very  asym- 
metrical, both  the  head  and  caudal  end  of  the  embryo  being  twisted  to  one  side, 
the  planes  which  would  be  true  for  the  body  of  the  embryo  in  the  region  of  the 
heart  would  not  be  true  elsewhere.  For  the  practical  use  of  the  student,  there- 
fore, in  these  younger  stages  it  is  better  to  determine  the  direction  of  the  plane 
by  the  floor  of  the  fourth  ventricle,  so  that  by  "transverse"  will  be  understood 
a  plane  of  section  which  cuts  the  head  of  the  embryo  symmetrically,  no  matter 
how  it  may  cut  the  body,  and  which  runs  parallel  to  the  floor  of  the  fourth  ven- 
tricle (medulla  oblongata).  The  frontal  plane  should  be  perpendicular  to  this 
and  also  cut  the  head  of  the  embryo  symmetrically.  The  sagittal  plane  in  these 
cases  is  also  that  of  the  head  and  not  of  the  body.  Such  planes  are  recom- 
mended because  in  the  study  of  the  sections  more  is  gained  by  having  the  planes 
readily  understood  in  the  region  of  the  head  than  in  the  region  of  the  body.  In 
later  stages,  when  the  body  has  become  straighter,  the  difference  in  planes  for  the 
head  and  the  body  may  be  practically  left  out  of  consideration,  except  that  for 
the  heads  of  older  pigs  when  they  are  cut  alone — as  on  account  of  the  size 
of  the  body  is  often  desirable — the  frontal  plane  is  chosen  so  as  to  run  at  right 
angles  to  the  plane  of  the  palate  and  symmetrically  through  the  embryo.  Sec- 
tions through  the  head  at  right  angles  to  this  may  be  designated  as  horizontal.* 
Students  will  find  that  it  is  very  much  easier  to  study  transverse  and  frontal  sec- 
tions when  they  are  symmetrical.  No  pains,  therefore,  should  be  spared  to  orient 
the  embryo  properly  in  the  microtome  before  the  sections  are  cut. 

*  The  system  of  planes  here  described  is  that  adopted  for  the  Harvard  Embryological  Collecti6n,  and 
has  been  found  convenient  in  practice. 


EXTERNAL  FORM  OF  EMBRYO  OF  7.5  MM.  221 

Selection  of  the  Stages. — The  most  profitable  stage  to  study  is  that  of  an  embryo 
of  from  ii  to  13  mm.  in  length.  Each  student  should  have  three  specimens 
of  this  stage,  and  it  is  advantageous  that  the  specimens  given  each  student  be 
approximately  of  the  same  size.  The  embryos  ought  to  be  first  studied  carefully 
as  to  their  external  form  and  then  cut  into  serial  sections  in  the  transverse,  sagit- 
tal, and  frontal  planes.  Of  these,  the  transverse  series  forms  the  principal  basis 
of  study,  and  the  other  series  are  to  be  used  principally  to  clear  up  the  student's 
conception  of  the  relation  of  parts.  Embryo  pigs  of  the  size  specified  have  the 
typical  class  characteristics  of  mammalian  embryos,  and  may  readily  be  distin- 
guished from  the  embryos  of  any  other  class  of  vertebrates.  The  differentiation 
of  the  anlages  of  all  the  important  organs  is  accomplished,  so  that  these  anlages 
can  be  identified  with  certainty  and  their  genetic  relations  to  the  adult  structures 
can  be  clearly  grasped  by  the  student.  At  the  same  time,  although  the  ana- 
tomical differentiation  is  well  advanced,  the  histological  differentiation  has  made 
very  little  progress,  hence  the  embryos  in  question  are  particularly  instructive  to 
beginners.  The  anatomy  of  the  pig  at  this  stage  is,  therefore,  readily  understood 
by  the  student  who  knows  the  general  anatomy  of  the  adult.  Older  embryos 
are  more  complicated  and  yield  such  long  series  of  sections  that  the  beginner 
is  apt  to  be  discouraged.  Younger  embryos,  owing  to  their  spiral  twisting,  are 
exceedingly  difficult  for  students"  to  understand  when  sectioned.  After  having 
thoroughly  mastered  the  structure  of  the  pig  embryo  of  from  11  to  13  mm.,  the 
student  may  advantageously  extend  his  study  of  embryos  to  other  sizes.  If,  as 
is  done  in  this  work,  the  principal  study  is  made  with  embryos  of  12  mm.,  the 
student  may  proceed  to  make  sections  of  other  stages  as  follows: 

Pig  embryo  of  6  mm.,  transverse. 

Pig  embryo  of  9  mm.,  transverse  and  sagittal  series. 

Pig  embryo  of  17  mm.,  transverse  series. 

Pig  embryo  of  20  mm.,  transverse  and  sagittal  series. 

(Of  the  head  alone,  the  frontal  series.) 
Pig  embryo  of  24  mm.,  of  the  head  alone,  frontal  series. 

The   Study   of   the  External  Form. 

The  student  should  make  a  careful  and  thorough  study  of  the  external  form 
of  every  embryo,  and  make,  with  the  aid  of  the  camera  lucida,  an  exact  drawing 
of  every  embryo  before  he  cuts  it  into  sections.  He  will  soon  learn  that  such  a 
drawing  is  almost  indispensable  for  the  interpretation  of  the  sections. 

In  the  following  paragraphs,  embryos  of  7.5,  10,  15,  and  20  mm.  are  figured 
and  described  from  specimens  which  have  been  hardened  in  Zenker's  fluid  and 
preserved  in  alcohol.  The  description  of  these  stages  will  be  sufficient  to  enable 
the  student  to  understand  any  of  the  embryos  he  is  required  to  study. 

Pig  Embryo  of  7.5  mm.  (Fig.  165).— The  student  maybe  helped  in  the  iden- 
tification of  parts  by  comparison  with  figure  166,  which  has  explanatory  lettering. 


222 


STUDY  OF  PIG  EMBRYOS. 


The   length   of   the   embryo   measured   in   a   vertical    line   as    the   embryo   is   placed 
in  the  figure  is  7.5  mm.,  but  its  greatest  length  in  any  direction  is  8.0  mm. 

The  head  is  somewhat  triangular  in  form,  being  broadest  toward  the  front  (the 
left  in  the  figure)  and  narrowing  posteriorly  to  join  the  rest  of  the  body.  The 
upper  boundary  of  the  head  is  a  nearly  straight  line,  the  extent  of  which  marks 
approximately  the  territory  of  the  hind-brain. 

Toward  the  left  the  outline  forms  a  rounded  curve  which  marks  the  territory 
of  the  mid-brain,  and  then  continues  obliquely  downward  in  a  straighter  course  un- 
til it  curves  over  on  to  the  under  side  where  it  forms  three  notches.  The  first 

notch  indicates  the  position  of  the  mouth,  the 
second  marks  the  boundary  between  the  first  and 
second  branchial  arches,  the  third  the  boundary 
between  the  second  and  third  arches.  On  the  tip 
of  the  head,  just  in  front  of  the  mouth,  is  a  shal- 
low depression,  the  anlage  of  the  nasal  pit,  and 
above  is  the  small  eye.  From  the  eye  to  the  mouth 
runs  a  shallow  furrow,  the  lachrymal  groove.  The 
first  branchial  arch  is  called  the  mandibular;  it  is 
broad  and  separated  by  a  furrow  from  the  second. 
Between  it  and  the  eye  lies  the  maxillary  process. 
The  second  branchial  arch  is  termed  the  hyoid. 
The  third  is  smaller  and  somewhat  drawn  inward, 
while  the  fourth  and  fifth  have  sunken  so  far  as  to 
produce  a  deep  pit  with  a  triangular  outline,  which 
has  been  named  the  cervical  sinus. 
The  body  has  a  long  curving  dorsal  outline  terminating  in  the  recurved  tail. 
Near  this  outline  thirty-seven  segments  show  externally,  because  each  one  creates 
a  protuberance  of  the  ectoderm.  The  least  developed  segments  are  in  the  tail. 
From  there  toward  the  head  they  show  a  progressive  advance  in  the  stage  of  devel- 
opment attained.  The  two  limbs  are  rounded  buds,  the  anterior  being  the  larger, 
and  offer  no  trace  of  their  future  articulation.  Between  them  stretches  a  long 
protuberance,  which  is  due  to  the  Wolffian  body  or  mesonephros,  the  precocious 
development  of  which  is  characteristic  of  ungulates.  In  man  at  a  corresponding 
stage  the  mesonephros  is  relatively  less  voluminous.  Immediately  ventrad  from  the 
fore-limb  the  two  lobes  of  the  liver  can  be  discerned  through  the  translucent  body- 
walls.  Between  the  liver  and  the  head  is  the  very  large  heart.  The  division  be- 
tween its  auricle  above  and  its  ventricle  below  can  be  seen  clearly.  The  abdominal 
region  of  the  body  is  prolonged  outward  between  the  tail  and  the  heart,  and  so 
forms  the  commencement  of  the  umbilical  cord,  the  end  of  which  is  marked  by 
a  thin  membrane,  the  amnion,  which  has  been  almost  completely  removed.  In 
life  the  amnion  forms  a  closed  sac  around  the  embryo,  and  is  distended  by  the 
amniotic  fluid.  From  the  end  of  the  umbilical  cord  project  remnants  of  the  yolk- 


FIG.  165. — PIG  EMBRYO  OF  7  .5  MM. 
X  8  diams. 


EXTERNAL  FORM  OF  EMBRYO  OF  10  MM. 


223 


sac,  and   the  allantois,  both  of   which   pass  through  the  cord   to   join  internal  struc- 
tures of  the  embryo. 

Pig  Embryo  of  10  mm.  (Fig.  166). — The  form  of  the  embryo  has  undergone 
notable  changes,  as  comparison  with  figure  165  will  show.  The  head  is  larger,  the 
expansion  in  the  regions  of  the  mid-  and  fore-brains  being  particularly  noticeable. 
The  limb-buds  have  lengthened,  as  has  also  the  umbilical  cord.  The  third  branchial 
arch  has  disappeared  from  the  surface  into  the  cervical  sinus.  The  head  as  a 
whole  lies  nearly  at  right  angles  with  the  back,  so  that  the  dorsal  outline  of  the 

Yen.     Md.       Au. 


C.S. 


A.L. 


Um 


M.L. 


P.L 


FIG.  166. — PIG  EMBRYO  OF  10  MM. 

A.L,  Anterior  limb.  Au,  Auditory,  or  first  gill-cleft.  C.S,  Cervical  sinus.  Md,  Mandibular  process.  M.L, 
Milk-line.  MX,  Maxillary  process.  N,  Nasal  pit.  Op,  Eye.  P.L,  Posterior  limb.  Seg,  Muscular  seg- 
ment. Um,  Umbilical  cord.  Ven,  Floor  of  fourth  ventricle  (medulla  oblongata).  X  8  diams. 

head  forms  a  distinct  though  rounded  angle  with  that  of  the  back.  This  angle 
marks  the  position  of  the  neck-bend,  and  also  the  junction  of  the  brain  with  the 
spinal  cord.  The  very  distinct  neck-bend  is  characteristic  of  the  mammalian  embryo. 
It  is  less  evident  in  birds  and  reptiles,  absent  in  amphibians  and  fishes.  Its  devel- 
opment probably  causes  the  cramping  of  the  ventral  cervical  region,  which  leads 
to  the  formation  of  the  cervical  sinus,  C.S,  and  to  the  disappearance  from  the 
surface  of  the  second,  third,  and  fourth  gill-clefts.  Another  consequence  of  the 
neck-bend  is  the  approximation  of  the  nasal  regions,  N,  of  the  head  to  the  cardiac 
region  of  the  body.  The  cephalic  region  has  a  second  flexure,  the  head-bend 


224  STUDY  OF  PIG  EMBRYOS. 

proper,  which  occurs  at  the  level  of  the  mid-brain,  the  nature  and  significance  of 
which  become  clearer  when  the  disposition  of  the  nervous  system  is  studied  (compare 
Fig.  178).  From  the  mid-brain  one  axis  extends  backward  through  the  region  of 
the  hind-brain,  Ven,  to  the  neck-bend;  the  other  axis  extends  vertically  downward 
to  the  region  of  the  fore-brain.  On  the  surface  of  the  head  we  find  the  nasal  pit, 
AT",  distinctly  marked.  The  eye,  Op,  shows  clearly  the  outlines  of  the  optic  vesicle 
and  of  the  lens  in  the  center.  It  is  entirely  without  lids.  The  small  size  of  the 
eye  is  a  characteristic  of  the  mammalian  embryo  by  which  it  differs  from  all  saurop- 
sidian  forms;  but,  as  previously  stated,  the  embryonic  eye  is  slightly  larger  in  certain 
other  mammals.  Below  the  eye  is  the  maxillary  process,  MX,  which  is  destined  to 
form  'the  greater  part  of  the  upper  jaw.  The  anterior  boundary  of  the  maxillary 
process  is  marked,  as  before,  by  the  lachrymal  groove,  which  runs  now  from  the 
angle  of  the  eye,  Op,  to  the  nasal  pit,  N.  The  mandibular  process,  Md,  out  of 
which  the  lower  jaw  is  to  be  developed,  is  separated  from  the  maxillary  process  by 
a  groove,  the  boundary  between  the  upper  and  lower  jaws,  and  is  bounded  .  behind 
by  a  second  groove,  Au,  the  anlage  of  the  future  meatus  auditorius  externus. 
This  groove  marks  the  boundary  between  the  mandibular  process  and  the  first,  or 
hyoid,  branchial  arch,  and  is  itself  the  ectodermal  member  of  the  first  gill-cleft. 
The  cavity  of  the  hind-brain  is  very  large  and  is  known  as  the  fourth  ventricle, 
Ven;  as  it  has  a  very  thin  roof  it  can  be  readily  distinguished.  The  thickened 
floor  of  the  fourth  ventricle  is  the  anlage  of  the  medulla  oblongata.  The  opening 
of  the  cervical  sinus,  C.S.,  is  triangular,  as  before;  within  it  are  hidden  the  third, 
fourth,  and  fifth  branchial  arches.  In  slightly  older  embryos  the  orifice  of  the 
sinus  is  further  contracted,  becoming  a  small  rounded  opening  which  finally  closes 
over  completely.  The  territory  of  the  mandibular  process  and  cervical  sinus  corre- 
sponds to  the  pharyngeal  region.  It  is  the  site  of  some  of  the  most  important, 
interesting,  and  complicated  developments  by  which  the  embryonic  is  changed  into 
the  adult  anatomy. 

The  dorsal  outline  of  the  body  forms  a  long  sweeping  curve,  ending  in  the 
tail.  Comparison  with  figure  165  shows  at  once  that  the  straightening  out  of  the 
dorsal  region  is  begun,  yet  at  this  stage  the  dorsal  side  of  the  embryo  is  nearly 
three  times  as  long  as  the  ventral.  The  umbilical  cord  has  grown  in  length,  and 
is  constricted  in  diameter  as  it  joins  the  abdomen,  yet  its  connection  with  the  body 
occupies  practically  the  entire  length  of  the  ventral  median  line.  The  position  and 
number  of  the  segments,  Seg,  is  still  shown  by  the  external  modeling.  Both  limbs 
are  well  advanced,  the  anterior,  A.L.,  more  so  than  the  posterior.  From  the  base 
of  the  brain  to  the  base  of  the  hind  limb  extends  the  milk-line,  M.L,  curving  so  as 
to  be  nearly  parallel,  to  the  dorsal  outline  of  the  body.  Along  it  the  mammary  glands 
are  ultimately  developed.  Extending  across  the  body  are  several  shadowy  lines 
shimmering  through  the  translucent  body-walls.  One  marks  the  position  of  the 
embryonic  diaphragm;  it  extends  from  the  upper  edge  of  the  anterior  limb  ob- 
liquely downward  toward  the  edge  of  the  umbilical  cord.  Another,  which  extends 


EXTERNAL  FORM  OF  EMBRYO  OF  15  MM. 


225 


in  a  nearly  straight  line  from  limb  to  limb,  marks  the  ventral  edge  of  the  large 
Wolffian  body,  or  mesonephros,  the  dorsal  limit  of  which  is  approximately  indicated 
by  the  milk-line,  M.L.  The  outlines  of  the  smaller  left  dorsal  lobe  of  the  liver 
are  distinct,  and  map  out  a  pointed  area  immediately  below  the  fore  limb,  A.L. 
Above  the  diaphragm  lies  the  protuberant  cardiac  region,  the  outlines  of  which  pass 
from  the  umbilical  cord  below  the  nasal  and  pharyngeal  regions  toward  the  cer- 
vical sinus.  The  long  tapering  tail  extends  alongside  the  umbilical  cord. 


FIG.  167. — PIG  EMBRYO  OF  15  MM.     X  8  diams. 

Pig  Embryo  of  15  mm.  (Fig.  167). — As  compared  with  figure  166,  the  present 
embryo  (Fig.  167)  has  not  only  grown  in  all  its  dimensions,  but  has  also  changed 
in  form.  Unlike  the  embryo  proper,  the  umbilical  cord  has  grown  very  little.  We 
notice  at  once  that  the  outline  of  the  back  is  less  curved  than  before,  that  the  ventral 
side  of  the  body  has  acquired  a  convex  outline,  and  that  the  head  has  become 
considerably  larger,  both  absolutely  and  relatively  to  the  body  of  the  embryo,  and 


226  STUDY  OF  PIG  EMBRYOS. 

has,  moreover,  risen  so  that  the  neck-bend  is  diminished.  The  limbs  are  beginning 
to  show  the  differentiation  of  the  feet.  Examined  more  carefully,  the  embryo  offers 
the  following  details:  the  eye,  which  is  characteristically  small,  has  become  almond- 
shaped,  and  the  circular  lens  can  be  seen  in  the  midst  of  it.  In  the  embryos  of 
rodents,  carnivores,  and  primates  the  eye  is  relatively  larger  than  in  the  pig.  By 
the  growth  of  the  facial  region  the  development  of  the  snout  has  been  initiated,  and 
the  opening  of  the  nasal  pit  now  appears  as  the  external  nares  toward  the  end  of 
the  short  snout.  The  lower  jaw  is  clearly  differentiated  and  the  slit  of  the  mouth 
is  distinct.  There  has  been  a  great  growth  of  the  regions  of  the  fore-brain  and 
mid-brain,  and  it  is  this  growth  chiefly  which  has  caused  the  relative  expansion  of 
the  head  as  compared  with  the  rest  of  the  body.  The  auditory  groove  now  ap- 
pears distinctly  as  the  anlage  of  the  external  meatus  of  the  ear,  behind  which  a 
protuberance  can  be  seen  which  is  the  anlage  of  the  external  concha  of  the  ear. 
The  cervical  sinus  has  wholly  disappeared.  Along  the  line  of  the  back  the  primitive 
segments  are  scarcely  recognizable  in  the  cervical  region,  but  near  the  upper 
limb  they  still  show  distinctly  and  from  there  are  indicated  with  increasing 
clearness  as  we  pass  toward  the  lower  limb.  The  marks  of  the  segmental  divisions 
do  not  extend  so  far  on  the  dorsal  side  as  in  the  earlier  embryos,  but  are  restricted 
to  what  may  be  called  the  segmental  ridge.  Along  the  milk-line  a  series  of 
small,  white,  circular  spots  can  be  seen.  In  the  specimen  figured  there  were  six 
of  these;  their  number  is  variable.  They  are  the  anlages  of  the  mammary  glands, 
and  are  at  this  stage  merely  local  thickenings  of  the  ectoderm  or  epidermis. 
There  has  been  a  considerable  growth  of  the  dorsal  region  of  the  body,  and 
this  is  perhaps  most  clearly  indicated  by  the  position  of  the  milk-line,  which  is 
much  farther  away  from  the  median  dorsal  line  than  in  the  10  mm.  pig.  Both 
limbs  are  paddle-shaped,  and,  though  still  very  short,  have  a  broad  terminal  ex- 
pansion, which  is  the  anlage  of  the  foot.  The  front  foot  has  somewhat  the  outline 
of  a  truncated  pyramid,  while  the  hind  foot  is  more  rounded.  In  the  anterior  limb 
the  differentiation  into  upper  and  lower  divisions  is  suggested. 

Pig  Embryo  of  20  mm.  (Fig.  168). — Comparison  of  this  stage  with  figure  167 
reveals  a  general  progress,  but  no  such  striking  changes  of  external  form  as  distin- 
guished the  embryo  of  15  mm.  from  that  of  10  mm.  Embryos  of  this  length  vary 
considerably  in  their  proportions,  but  the  one  figured  is  characteristic  of  the  stage. 
The  enormous  transverse  diameter  of  the  body  as  compared  with  its  length  is  very 
striking,  and  the  very  large  size  of  the  head  in  proportion  to  the  body  is  almost 
equally  remarkable.  In  the  head  the  growth  of  the  regions  of  the  fore-brain  and 
mid-brain  has  continued,  and  the  divisions  between  the  mid-brain  and  hind-brain 
are  marked  by  concavities  in  the  outline  of  the  head.  The  eye  is  both  absolutely 
and  relatively  larger.  Above  it  can  be  distinguished  readily  the  anlages  of  the  great 
bristles  which  develop  over  the  eye,  corresponding  to  the  human  eyebrow.  These 
anlages  appear  as  whitish  spots,  for  they  are  thickenings  of  the  ectoderm.  The 
snout  has  increased  in  length;  the  external  ear  has  grown  longer  and  has  begun  to 


EXTERNAL  FORM  OF  EMBRYO  OF  20  MM. 


227 


FIG.  168. — PIG  EMBRYO  OF  20  MM.     X  8  diams. 


228  STUDY  OF  PIG  EMBRYOS. 

assume  its  permanent  pointed  form.  The  limbs  have  increased  considerably  in 
length,  but  not  yet  enough  to  project  beyond  the  abdomen.  In  both  feet  the 
differentiation  of  five  toes  is  clearly  indicated.  The  milk-line,  as  a  line,  has  almost 
completely  vanished,  but  the  row  of  dots,  the  'anlages  of  the  mammary  glands, 
which  develop  along  the  milk-line,  persists"  and  will  undergo  further  development 
in  later  stages.  The  number  in  the  specimen  figured  is  five.  As  the  row  of 
anlages  marks  the  position  of  the  milk-line,  it  is  readily  seen  that  the  line  has 
migrated  ventralward  as  compared  with  earlier  stages.  Comparison  of  the  embryos 
of  15  and  20  mm.  demonstrates  that,  during  the  period  comprised  between  the  two 
stages,  the  growth  of  the  dorsal  part  of  the  embryo-  is  far  greater  than  of  the 
ventral  part.  Comparison  of  figures  167  and  168  shows  at  once  that  the  area 
occupied  in  both  figures  by  the  region  on  the  ventral  side  of  the  milk-line  is 
about  the  same.  In  the  pig  of  20  mm.  there  is  no  indication  of  the  segmental 
structures  recognizable  in  the  surface  modeling. 

Pig  Embryo  of  7.8  mm.     General  Anatomy. 

Anatomical  Reconstructions  from  the  Sections. — Reconstructions  are  of  the  greatest 
assistance  in  the  study  of  sections,  and  much  facilitate  the  identification  of  all  the 
parts.  Students  using  this  book  should,  while  examining  their  sections,  constantly 
refer  to  the  reconstructions.  It  is  unnecessary  to  give  elaborate  descriptions  of 
each  of  them,  since  the  explanations  of  the  lettering  of  the  figures  will  suffice  for 
the  identification  of  all  the  parts  shown.  Certain  brief  explanations  as  to  each  of 
the  figures  are,  however,  desirable.  The  chief  value  of  reconstructions  is  to  render 
clear  the  topographical  distribution  of  the  organs. 

The  reconstruction*  presented  in  figure  169  shows  the  general  topography  of  the 
embryo,  and  illustrates  chiefly  the  digestive,  vascular,  and  central  nervous  systems. 
The  digestive  tract  is  represented  by  the  outline  of  its  epithelial  portion  only.  In 
the  actual  specimen  the  walls  are  thicker,  because  they  include  the  mesenchyma 
immediately  surrounding  the  epithelium.  The  mouth  is  an  opening  between  the 
head  and  mandible,  Md,  and  leads  directly  into  the  pharynx,  Ph.  From  the  dorsal 
side  of  the  mouth  springs  the  hypophysis,  Hy,  which  lies  close  against  the  wall  of 
the  fore-brain  and  is  destined  to  form  the  anterior  lobe  of  the  pituitary  body. 
The  pharynx,  Ph,  is  narrow  in  its  dorso-ventral  diameter,  and  is  represented  in 
median  section.  From  its  dorsal  surface  arises  the  small  conical  diverticulum, 
S.P,  situated  near  the  hypophysis.  If  followed  backward,  the  pharynx  is  found  to 
bend  tailward  and  to  form  two  median  branches,  the  more  dorsal  of  which  is  the 
oesophagus,  (E.  The  ventral  branch  is  the  trachea  which  soon  bifurcates  to  form 
the  main  bronchi,  of  which  the  right  only  is  shown  in  the  figure,  at  Bro.  The 
lateral  gill-pouches  are  not  shown  in  this  figure.  The  oesophagus  has  lengthened, 
and  leads  to  the  stomach,  St,  which  has  already  descended  and  has  so  revolved 

*  Through  the  kindness  of  Dr.  Thyng,  it  is  possible  to  use  this  illustration  in  advance  of  its  publication 
by  the  author,  whose  paper  on  the  anatomy  of  the  7  .8  mm.  pig  is  soon  to  appear  in  full. 


GENERAL  ANATOMY  OF  EMBRYO  OF  7.8  MM. 


229 


Car.  in         Md 
Isth       A. has     S.P   \     H.B  Car. ex     Ao.III 


Ao.IV 


Nch 


Ao 


A.il 


V.om 


A.o.m     V  .port 


FIG.  169. — PIG  EMBRYO  OF  7.8  MM.     TRANSVERSE  SERIES  1358.     RECONSTRUCTION  BY  FREDERICK  W.  THYNG. 

a,  Artery.  A  .has,  Arteria  basilaris.  A  .cand,  Caudal  artery.  A  .il,  Doifble  openings  of  left  common  iliac  artery 
into  the  aorta.  All,  Allantois.  Ao,  Main  aorta.  Ao.D,  Descending  aorta.  Ao.III,  Right  third  aortic 
arch.  A o.I V,  Right  fourth  aortic  arch.  Ao.V,  Right  fifth  aortic  arch.  A . o.m,  Omphalo-mesaraic  artery. 
A  Mm,  Umbilical  artery.  Bro,  Bronchus.  Car,  Carotid  loop.  Car. ex,  External  carotid  anlage.  Car. in, 
Internal  carotid.  Cce.ax,  Coeliac  axis.  Clo,  Cloaca.  Col,  Colon,  entodermal  part.  F.B,  Fore-brain. 
For.ia,  Interatrial  foramen.  For  .iv,  Foramen  interventriculare.  F.  ov,  Foramen  ovale.  H.B,  Hind-brain. 
Hy,  Hypophysis.  In.caud,  Caudal  intestine.  II,  Small  intestine,  entodermal  part.  Isth,  Isthmus.  Ki, 
Kidney.  Li,  Liver.  M.B,  Mid-brain.  Md,  Mandible.  Nch,  Notochord.  N.cerv,  Cervical  nerve.  CE,  (Esopha- 
gus. Pan.d,  Dorsal  pancreas.  Pan.v,  Ventral  pancreas.  (The  line  does  not  quite  reach  the  organ, 
which  lies  ventrad  from  the  intestine.)  Ph,  Pharynx.  Som,  Somatopleure.  S.P,  Seessel's  pocket.  Sp.c, 
Spinal  cord.  St,  Stomach.  Ve.hep,  Hepatic  vein.  V.om,  Omphalo-mesaraic  vein.  V.port,  Portal  vein. 
V.pul,  Pulmonary  vein.  V.s.m,  Superior  mesenteric  vein.  W.D,  Wolffian  duct.  Yk,  Yolk-sac.  X  14 
diams. 


230  STUDY  OF  PIG  EMBRYOS. 

on  its  own  axis  that  its  dorsal  border  nearly  faces  the  left.  Just  below  the  en- 
trance of  the  oesophagus,  the  cardiac  end  of  the  stomach  has  a  small  diverticulum, 
which  is  characteristic  for  the  pig,  but  does  not  occur  in  man.  Below  the  stom- 
ach the  digestive  canal  is  narrow  and  forms  a  long  loop,  II,  which  extends  t6  the 
umbilical  opening,  where  it  joins  the  neck  of  the  yolk-sac,  Yk.  This  portion  of 
the  tract  gives  rise  to  the  duodenum,  jejunum,  and  most  of  the  ileum.  Beyond 
the  yolk-sac  .  the  intestinal  tract  is  continued  as  a  narrow  tube,  Col,  which  leads  to 
a  considerable  expansion,  the  cloaca,  Clo,  at  the  base  of  the  tail.  From  the  cloaca 
there  -extends  into  the  tail  a  very  narrow  prolongation  of  the  entodermal  canal 
known  as  the  tail-gut,  or  caudal  intestine,  In.caud.  Into  the  cloaca  also  open  the 
Wolffian  ducts,  W.D,  which  are  the  .ducts  of  the  primitive  excretory  organs  of  the 
embryo.  The  duct  on  the  left  side  is  represented  as  cut  off  close  to  the  cloaca. 
More  of  the  right  duct  is  included  in  order  to  show  the  anlage  of  the  true  or  per- 
manent kidney,  Ki,  which  is  budding  off  from  it.  Returning  to  the  portion  of  the 
intestine  (duodenum)  next  to  the  stomach,  we  find  clearly  displayed  the  anlage  of 
the  dorsal  pancreas,  Pan.d.  The  anlage  of  the  ventral  pancreas  also  appears,  Pan.v, 
but  less  clearly.  It  is  an  elongated  mass  of  entoderm,  lying  to  the  right  of  the 
duodenum  and  ventral  to  the  portal  vein,  V.port.  It  takes  its  origin  from  the  duct 
of  the  liver.  The  liver  itself,  Li,  has  already  acquired  a  considerable  size.  On 
its  ventral  surface  lies  the  gall-bladder,  which  is  connected  with  the  liver  substance 
by  .several  cords  of  hepatic  cells.  In  the  adult,  only  one  connection  between  the 
gall-bladder  and  the  liver  persists. 

The  central  nervous  system  is  represented  as*  seen  in  median  section,  display- 
ing the  cavity  and  the  inner  surfaces  of  the  walls  of  the  cavity.  In  the  region  of 
the  head  the  tube  is  already  much  dilated  to  make  the  brain.  In  the  region  of 
the  body  it  narrows  to  form  the  spinal  cord,  Sp.c,  which  gradually  diminishes  in 
diameter  toward  the  tail.  The  brain  is  clearly  subdivided  into  its  three  primary 
vesicles:  the  fore-brain,  F.B;  the  mid-brain  M.B;  and  the  hind-brain,  H.B.  Owing 
to  the  head-bend,  the  mid-brain  forms  an  arch.  It  is  less  in  diameter  than  either 
mid-brain  or  hind-brain.  The  demarcation  between  the  mid-brain  and  hind-brain, 
known  as  the  isthmus,  Isth,  is  strongly  marked.  The  hind-brain,  H.B,  is  longer 
than  the  mid-  and  fore-brain  combined.  It  diminishes  in  diameter  posteriorly,  and 
passes  without  definite  demarcation  into  the  spinal  cord.  Owing  to  the  head-bend, 
the  fore-brain,  F.B,  is  brought  to  underlie,  as  it  were,  the  anterior  portion  of  the 
hind-brain,  and  to  overlie  the  heart.  The  heart  is  a  very  large  organ,  which  is 
represented  in  the  figure  somewhat  to  the  left  of  the  median  line.  It  consists  of  a 
smaller  dorsal  or  upper  chamber — the  atrium  or  auricle,  and  a  lower  larger  cham- 
ber— the  ventricle.  The  auricle  shows  two  openings  by  which  it  communicates  with 
the  right  auricle.  The  upper  of  these,  F.ov,  is  the  foramen  ovale;  the  lower  is 
the  so-called  interatrial  foramen,  For.ia.  The  small  pulmonary  vein,  V.pul,  opens 
directly  into  the  auricle.  The  ventricle  has  a  trabecular  or  sinusoidal  structure. 
The  cavity  in  the  drawing  is  that  of  the  left  side.  It  communicates  with  the  cavity 


GENERAL  ANATOMY  OF  EMBRYO  OF  12  MM.  231 

of  the  right  ventricle  by  means  of  the  interventricular  foramen,  For.iv.  Above 
the  ventricle  is  seen  the  main  trunk  of  the  aorta,  which  divides  and  gives  rise  to 
three  aortic  arches,  Ao.III,  Ao.IV,  Ao.V.  The  first  and  second  arches  are  still 
present,  but  are  very  slender.  Their  common  stem,  Car.ex,  forms  the  future  stem 
of  the  external  carotid.  From  the  dorsal  end  of  the  third  arch,  Ao.III,  there  runs 
forward  the  vessel,  Car.in,  which  joins  the  first  and  second  arches,  and  itself  be- 
comes part  of  the  stem  of  the  internal  carotid.  The  aortic  arches  form  on  each 
side  of  the  descending  aorta,  Ao.D,  which  unites  with  its  fellow  just  above  the  level 
of  the  stomach  to  form  the  main  median  dorsal  aorta,  Ao.  The  aorta  gives,  off  a 
series  of  intersegmental  vessels  which  mount  dorsalward  between  the  spinal  cord 
and  the  outer  surface  of  the  embryo.  The  main  aorta  gives  off  on  its  ventral  side 
a  vessel,  Cce.ax,  which  becomes  the  cceliac  axis  of  the  adult;  and  lower  down,  three 
vessels,  A.o.m,  which  anastomose  in  several  places.  In  their  course  to  the  yolk-sac 
they  cross  to  the  right  of  the  intestine  and  give  off  branches  to  the  mesentery.  The 
two  upper  roots  atrophy  later,  but  the  lower  persists  to  form  the  stem  of  the 
superior  mesenteric  artery.  Just  before  reaching  the  cloaca  the  aorta  gives  off 
vessels,  A.il}  which  run  to  the  hind  leg,  and  develop  into  the  external  iliac  of 
the  adult.  Just  beyond,  the  aorta  gives  off  a  small  branch,  A.caud,  to  the  tail;  and 
at  the  same  time  bifurcates  to  form  the  umbilical  arteries,  A.um,  which  take  a 
sinuous  course  alongside  the  allantois,  All,  and  ultimately  ramify  outside  of  the 
body  of  the  embryo  in  the  walls  of  the  allantois.  The  allantois  itself  springs  from 
the  cloaca  as  a  narrow  canal  which  makes  a  sharp  bend  and  runs  to  the  umbilicus, 
gradually  expanding.  Outside  of  the  embryo,  the  allantois  forms;  in  the  pig — as 
in  all  ungulates— a  very  large  vesicle.  In  man  and  the  primates,  on  the  contrary, 
the  allantois  is  rudimentary.  This  is  one  of  the  most  striking  differences  between 
the  pig  and  the  human  embryo. 

Of  the  veins  little  is  shown  in  this  figure,  but  the  omphalo-mesaraic  and  portal 
veins  are  included.  The  former,  V.om,  arises  on'  the  surface  of  the  yolk-sac, 
passes  through  the  umbilicus  into  the  mesentery  of  the  intestine,  and  extends 
parallel  with  the  ileum  until  it  crosses  it,  a  little  posterior  to  the  pancreas.  It  is 
there  joined  by  another  vein,  V.s.m.  the  superior  mesenteric,  which  is  of  about 
the  same  caliber.  The  common  trunk  formed  by  the  union  of  these  two  veins  is 
the  portal  vein,  V .port,  which  divides  into  two  branches,  extending  to  the  right  and 
left  of  the  omphalo-mesaraic  vessels.  The  left  branch  is  small.  It  passes  on  the 
left  of  the  dorsal  pancreas  and  extends  to  the  liver.  The  right  branch  passes 
between  the  two  pancreatic  anlages,  then  bends  to  the  right  and  enters  the  liver, 
into  the  sinusoids  of  which  it  discharges  its  blood-stream. 

Pig  Embryo  of  12.0  mm.     General  Anatomy. 

Dissection  of  the  Viscera  (Fig.  170). — The  specimen  figured  measured  12.7 
mm.  Both  limbs  and  the  body-wall  on  the  left  side  have  been  removed,  displaying 
the  organs  in  situ.  The  umbilical  cord  has  been  cut  lengthwise  so  as  to  display  its 


232 


STUDY  OF  PIG  EMBRYOS. 


internal  structure.  The  body  proper  is  divided  by  the  diaphragm,  Dia,  into  an 
upper  smaller  pericardial  chamber  and  a  lower  larger  abdominal  chamber.  The 
diaphragm  is  a  thin  membrane,  which  extends  from  the  level  of  the  base  of  the 
fore  leg,  F.L,  to  the  ventral  wall  of  the  body.  The  body  seems  filled  chiefly  by 
three  large  organs:  the  heart,  Au,  Ven,  above  the  diaphragm;  and  the  liver,  Li, 


Yen 


Dia 


Urn' 
Yk.s 

CCBC 

COB 

Col 

II 


C:s 


Au 


-     F.L 


Li1 


Li 


W.B 


AU 


Clo 


H.L 


Soni 


FIG.  170. — PIG  OF  12.7  MM.     DISSECTION  OF  THE  VISCERA  BY  RICHARD  E.  SCAMMOX. 

AU,  Allantois.  Au,  Auricle  of  the  heart.  Ccec,  Caecum.  Clo,  Cloaca.  Coe,  Ccelom.  Col,  Large  intestine. 
C.s,  Cervical  sinus,  nearly  obliterated.  Dia,  Diaphragm.  F.  L,  Front  limb.  H.  L,  Hind  limb.  //,  Small 
intestine.  Li,  Ventral,  Li1,  dorsal  lobe  of  liver.  Som,  Cut  surface  of  somatopleure.  Um,  Wall  of  the  um- 
bilical cord.  Ven,  Ventricle  bf  the  heart.  W.B,  Wolffian  body.  Yk.s,  Yolk-stalk.  X  8  diams. 

Li1,  and  Wolffian  body,  W.B,  below  the  diaphragm.  The  heart  shows  its  large  au- 
ricle, Au,  the  walls  of  which  are  thin  and  translucent.  It  entirely  conceals  the  veins, 
which  enter  the  heart  through  the  diaphragm,  and  the  aorta,  which  runs  from  the 
ventricle  toward  the  pharynx.  The  ventricle,  Ven,  is  much  larger  than  the  auricle; 
its  walls  are  not  translucent;  its  rounded  apex  points  away  from  the  auricle.  The 
liver  lies  close  against  the  diaphragm  and  shows  two  lobes:  the  larger  ventral  lobe, 


234 


STUDY  OF  PIG  EMBRYOS. 


FIG.  171. 


.  \ 


FIG.  172. — PIG  EMBRYO  OF  12.0  MM.     RECONSTRUCTION  FROM  THE  TRANSVERSE  SECTIONS,  SERIES  5. 

For  the  most  part  the  organs  represented  are  in  or  near  the  median  plane.     The  drawing  illustrates  especially 
the  .disposition  of  the  alimentary  tract,  the  arterial  system,  and  the  heart.     A. has,  Basilar  artery,  formed  by 
the  union  of  the  two  vertebral  arteries,  A  .v,  and  joined,  under  the  mid-brain,  by  the  anterior  ends  of  the  internal 
carotids.     A.cau,  Caudal  artery,  the  small  median  prolongation  of  the  dorsal  aorta.     All,  The  allantois; 
it  is  joined  by  the  Wolffian  duct,  and  empties  into  the  cloaca,     an.pl,  The  cloacal  membrane.     Ao,  Median 
dorsal  aorta.     Ao.d,  Right  descending  aorta;  a  small  vessel  connecting  the  dorsal  ends  of  the  third  (carotid) 
and  fourth  aortic  arches.     Ao.D,  Main  right  descending  aorta,  passing  downward  to  join  its  fellow  and  form 
the  median  aorta.     A.s,  Subclavian  artery.     Au,  Left  auricle  of  the  heart.     A.unt,  The  umbilical  artery 
which  runs  in  the  mesodermic  wall  of  the  allantois  and  joins  the  caudal  end  of  the  dorsal  aorta.     A  .v,  Vertebral 
artery.    "A.vi,  The  vitelline  artery,  which  becomes  the  superior  mesenteric  artery  of  the  adult,     c,  Artery 
known  as~lh'e  cceliac  axis  in  the  adult,     car.e,  External  carotid  artery,  arising  from  the  base  of  the  third  aortic 
-    arch.     car. i,  Internal  carotid  artery.     Cbl,  Cerebellum,     d,  Left  duct  of  Cuvier.     Dien,  Diencephalon.     D.V, 
Ductus  venosus  Arantii.     ep,  Small  plug  of  entodermal  epithelium  in  the  rectum;  there  are  several  small 
irregular  passages  through  the  plug.     Epgl,  Anlage  of  the  epiglottis.    /,  Interventricular  foramen,  opening  into 
the  space  a,  b,  c,  of  Fig.  176.     F.  M,  Foramen  of  Monro.    f.o,  Foramen  ovale,  between  the  cardiac  auricles, 
"-bladder.     In,  Entodermal  wall  of  the  intestine,     is,   One  of  the  series  of  intersegmental  arteries, 
^'ind  end  of  the  ureter,  the  anlage  of  the  renal  pelvis.    Lar,  Anlage  of  the  larynx,  consisting 
°lial  plate.    Li,  Liver.    Lu,  Entodermal  portion  of  the  lung.     M.b,  Mid-brain-     Md.ob, 
>7\  A  spinal  nerve.     Neu,  Neuromeres  of  the  hind-brain.     Oe,  (Esophagus,     op,  Stalk. 
•Ik  is  the  anlage  of  the  optic  nerve.     P. A,  Pulmonary  artery,     pan,  Dorsal  anlage 
art  of  which  becomes  the  duct;  behind  the  duct  is  the  anlage  of  the  ventral  pan- 
>wth  from  the  duct  of  the  gall-bladder,     p.c,  Pericardia!  cavity.     P.V,  Pul- 
SV,  Stomach;  the  letters  are  placed  beside  the  diverticulum  which  is  char- 
Mi  man.     t,  Posterior  portion  of  the  tongue,  an  eminence  on  the  floor  of 
•  >:ie  ventral  ends  of  the  third  and  fourth  gill-arches.     T.i,  Tuberculum 
i  of  the  tongue.     Tra,  Entodermal  trachea.     Ug.si,  Cloaca.     Um.d, 
icle,  the  letters  being  placed  on  the  septum  inferius.     V.mes,  Superior 
.7';,  Vitelline  vein.     W.Dj  Wolffian  duct,     x,  Anastomosis  between 
ins.      Yk,  Yolk-sac.      Yk.s,  Entodermal  yolk-stalk,  connecting  the 
^-(Draum  by  F.  T.Lewis.) 


GENERAL  ANATOMY  OF  EMBRYO  OF  12  MM. 


235 


as. 


FIG.  172. 


236  STUDY-  OF  PIG  EMBRYOS. 

Li,  extends  to  the  umbilicus;  the  smaller  dorsal  lobe,  Li1,  abuts  against  the  Wolffian 
body.  In  the  fresh  specimen  the  liver  is  conspicuous  by  its  dark  color.  The 
Wolffian  body,  W.B,  is  very  large;  it  extends  from  the  dorsal  edge  of  the  dia- 
phragm to  the  level  of  the  hind  limb,  H.L,  or,  in  other  words,  to  the  pelvic  end 
of  the  abdomen.  The  Wolffian  tubules  can  be  seen  running  transversely  just 
within  the  surface  of  the  organ  and  nearly  parallel  to  one  another.  The  large 
size  of  the  Wolffian  body  (fetal  kidney  or  mesonephros)  is  characteristic  of  all 
known  amniote  embryos.  The  umbilical  cord  projects  upward  from  the  ventral 
wall  of  the  abdomen;  its  cut  surfaces,  Um,  are  indicated  by  parallel  lines.  The 
abdominal  cavity  extends  into  the  cord,  forming  the  umbilical  coelom,  Cc&.  From 
underneath  the  liver  and  on  the  right  side  of  the  embryo,  the  small  intestine,  //, 
runs  out  into  the  umbilical  coelom,  makes  a  sharp  loop  turn,  and  passes  over  into 
the  large  intestine,  Col,  which  runs  back  to  the  abdomen  on  the  left  side  of  and 
nearly  parallel  to  the  ileum,  //;  it  passes  under  the  tip  of  the  liver,  then  between 
the  two  Wolffian  bodies,  where  it  curves  in  the  median  plane — though  this  is  not 
shown  in  the  figure — and,  bending  tailward,  terminates  in  the  cloaca,  Clo.  From 
the  tip  of  the  intestinal  loop  springs  the  stalk,  Yk.s,  of  the  yolk-sac.  The  begin- 
ning of  the  large  intestine  is  marked  by  a  small  knob,  CCKC,  the  anlage  of  the 
csecum.  At  this  stage  the  small  and  large  intestines  are  of  about  the  same  di- 
ameter. From  the  cloaca,  Clo,  a  hollow  prolongation,  All,  runs  out  into  the  caudad 
wall  of  the  umbilical  cord;  it  is  the  stalk  of  the  allantois. 

Anatomical  Reconstructions  from  the  Sections. — Six  reconstructions  of  the  anatomy 
of  this  stage  are  figured.*  Figures  171,  173,  174,  175,  177,  and  179  are  based 
on  the  same  series  which  has  supplied  the  transverse  section  of  the  12  mm.  pig 
figured  in  the  following  pages. .  The  umbilical  cord  of  this  embryo  (Series  5) 
having  been  damaged,  the  loop  of  the  intestine  in  the  umbilical  cord  has  been 
added  to  figures  171  and  174  by  a  reconstruction  from  another  series  (No.  518)  of 
an  embryo  of  the  same  size. 

The  following  remarks  call  attention  to  some  of  the  more  important  anatomical 
relations  shown  by  the  reconstructions.  The  great  volume  of  the  central  nervous 
system  as  compared  with  the  remaining  parts  is  very  striking.  Of  the  other 
organs,  the  three  which  are  most  conspicuous  by  their  size  are  the  heart,  liver,  . 
and  Wolffian  bodies.  Another  striking  peculiarity  of  the  embryo  is  the  great 
diameter  of  the  blood-vessels,  and  especially  of  the  veins,  which  are  of  relatively 
enormous  diameter,  being  proportionately  much  larger  than  in  the  adult.  In 
marked  contrast  with  this  is  the  small  diameter  of  the  cavity  of  the  trachea  and 
lungs  and  of  the  entire  intestinal  canal. 

Figure    172   represents   in    the   main   a   median   section    of   the   embryo   together 


*  Figures  171,  175, 177,  and  179  were  made  by  Dr.F.  T.  Le\v;s;  figure  174  by  Mr.  P.P.  Johnson;  figure  173 
is  from  a  wax  reconstruction  by  Dr.  John  L.  Bremer.  In  all  cases  the  reconstructions  were  made  with  special 
reference  to  their  present  use.  It  gives  me  much  pleasure  to  acknowledge  my  obligations  to  these  members  of 
our  laboratory  staff. 


GENERAL  ANATOMY  OF  EMBRYO  OF  12  MM. 


237 


with  the  organs  of  the  right  side,  but  with  two  exceptions,  first,  the  floor  of  the 
pharynx  is  represented  as  if  cut  through  considerably  to  the  left  of  the  median 
plane;  second,  the  heart  is  cut  to  the  left  of  the  median  plane.  The  brain  and 
spinal  cord  are  drawn  as  if  opened  to  show  the  modeling  of  the  inner  surface  of 
the  medullary  tube.  The  pharynx  is  so  drawn  as  to  indicate  something  of  the 
modeling  of  its  floor  surface.  The  opening  of  the  veins  into  the  heart  and  of  the 
auricle  into  the  ventricle,  and  the  interventrieular  orifices,  are  shown.  Of  the 
digestive  canal  only  the  entoderm  is  represented,  so  that  the  figure  displays  the 
entodermal  walls  of  the  oesophagus,  stomach,  and  intestine,  and  shows  the  two 
pancreatic  anlages.  Similarly  only  the  entodermal  portion  of  the  trachea  and 


Bu — 


M 


Oe     Tra    IV 


III 


FIG.  173. — PIG,  12  MM.     SAGITTAL  SERIES  7.     WAX  RECONSTRUCTION  BY  J.  L.  BREMER. 

Bu,  Pharyngeal  bursa.     Hy,  Hypophysis.     M,  Mouth-cavity.     Oe,  (Esophagus.     Tra,  Trachea.     I,  II,  III,  IV, 
Gill-pouches,  developed  as  lateral  pouches  of  the  pharynx.     X  40  diams. 

lungs  is  included,  -and  the  same  is  true  of  the  caudal  end  of  the  Wolffian  duct 
and  of  its  outgrowth,  which  forms  the  anlage  of  the  kidney.  The  same  is  further 
true  of  the  gall-bladder,  of  which  only  the  epithelial  portion  is  represented.  In 
this  figure  the  arterial  system  is  fully  displayed.  The  pulmonary  artery  and  the 
aortic  trunk  are  completely  separated.  A  small  artery  from  the  pulmonary  arch 
to  the  lungs  is  included,  and  the  figure  shows  the  entire  system  of  branches  from 
the  main  aorta. 

Figure  175  is  in  many  respects  similar  to  figure  172,  and  is  intended  to  show 
chiefly  the  disposition  of  the  veins.  There  are  also  included  in  this  figure  the 
Wolffian  body  and  its  duct.  The  pharynx  and  the  heart  are  supposed  to  have 
been  cut  through,  well  to  the  right  of  the  median  plane.  This  makes  it  possible 
to  indicate  in  the  figure  the  origin  of  the  pulmonary  aorta  and  of  the  true  aorta. 
The  following  are  the  most  important  veins:  the  umbilical,  which  passes  around 
the  umbilical  opening  and  enf2rs  the  liver;  the  portal  vein,  which  receives  the 


238 


STUDY  OF  PIG  EMBRYOS. 


Cl.iv 


blood  from  the  abdominal  viscera,  and  also  delivers  it  to  the  liver.  In  this  speci- 
men there  is  quite  a  wide  and  free  connection  within  the  liver  between  the  portal 
and  umbilical  veins.  In  other  embryos  of  this  size  such  a  connection  does  not 
always  exist.  The  large  vena  cava  inferior  is  on  the  right  side  of  the  embryo, 

and  passes  through  the  liver,  which  thus  receives 
blood  directly  from  the  Wolffian  bodies  and  the 
cardinal  veins.  From  the  upper  side  of  the  liver 
the  hepatic ,  vein  goes  directly  to  the  heart,  uniting 
with  the  common  cardinals,  which  receive  the 
jugular  veins  from  the  head  and  the  post-cardinal 
veins  from  the  body.  The  cardinal  veins  are  now 
very  much  changed.  In  earlier  stages  they  ex- 
tended from  the  common  cardinals  almost  the 
entire  length  of  the  embryo.  Of  this  great  vessel 
there  now  remains  connected  with  the  common 
cardinal  only  a  comparatively  short  vessel. 

Figure  173  gives  a  lateral  view  of  the  pharynx, 
in  order  to  show  the  four  gill-pouches,  I,  II,  III, 
IV,  as  seen  from  the  side.  The  curvature  of  the 
Pan.d  pharynx,  and  its  passage  at  its  posterior  end  into 
the  ventral  trachea  and  dorsal  oesophagus,  are 
clearly  shown.  As  regards  the  gill-pouches,  the 
first  rises  upward  and  terminates  in  a  sharp  apex; 
the  second  lies  nearly  in  the  same  plane  as  the 
portion  of  the  pharynx  from  which  it  arises  and 
has  a  prolongation  toward  the  third  pouch,  and 
the  end  of  the  prolongation  bends  ventralward; 
the  third  is  narrow  as  it  parts  from  the  pharynx, 
then  bends  downward  and  forward  and  has  a  pro- 
longation, the  anlage  of  the  thymus  gland,  which 
extends  toward  the  root  of  the  aorta;  the  fourth 


Pan.v 


Tra 


Lu 


Oe 


Si 


Col 


FIG.    174. — PIG    EMBRYO   OF    12.0   MM. 

RECONSTRUCTION    FROM    SERIES    5, 

TO  SHOW  THE  ENTODERMAL  CANAL, 

VIEWED  FROM  THE  VENTRAL  SIDE. 
(  'IP,    Caecum.      Cl.iv,   Fourth   gill-pouch. 

Col,     Colon.        Duod,     Duodenum. 

Hep,     Hepatic      duct.       77,    Ileum. 

La,  Larynx.  Lu,  Lung  Oe,  (Esoph-  also  begins  with  a  narrow  stalk  and  has  an  ex- 
panded end,  one  apex  of  which  extends  outward 
(to  join  the  cervical  sinus),  the  other  inward  and 
downward,  the  latter  being  the  anlage  of  the  post- 
branchial  body.  On  the  dorsal  side  projects  the 
pedunculate  hypophysis,  which  is  developed,  not 
from  the  pharyngeal  entoderm,  but  from  the  ectoderm  of  the  mouth-cavity  proper. 
Figure  174  is  inserted  in  order  to  give  a  clear  idea  of  the  entodermal  canal, 
as  viewed  from  the  ventral  side.  Only  the  posterior  end  of  the  pharynx  is  in- 
cluded, and  the  cloaca  and  the  allantois  are  omitted.  The  figure  represents  only 
the  entoderm  without  anv  of  the  surrounding  .mes'vlerm. 


agus.  Pan.d,  Dorsal  pancreas. 
Pan.v,  Ventra.1  pancreas.  St, 
Stomach.  Tra,  Trachea.  Tr.Br, 
Tracheal  bronchus.  I'k.s,  Yolk- 
sac.  X  14  diams. — (Drawn  by  F.  P. 
Johnson.) 


240 


STUDY  OF  PIG  EMBRYOS. 


. 


P'IG.  176. — PIG  EMBRYO  or  12.0  MM.     RECONSTRUCTION  FROM  THE  TRANSVERSE  SECTIONS,  SERIES  5. 

The  figure  illustrates  chiefly  the  veins  of  the  right  side,  and  shows  the  right  auricle  and  ventricle  of  the  heart. 
A,  The  right  umbilical  artery,  only  a  small  portion  being  drawn  as  it  curves  around  the  Wolffian  duct,  W.D. 
a,  Tip  of  aortic  septum,  which  divides  the  aortic  limb  of  the  heart  into  the  pulmonary  aorta,  P,  and  main 
aorta,  Ao;  by  a  growth  of  the  cardiac  tissue,  a,  b,  and  c  of  the  figure  become  joined,  shutting  off  the  space 
around  the  base  of  the  aorta;  this  space  communicates  by  the  interventricular  foramen  with  the  left  ventricle, 
and  serves  as  the  permanent  or  adult  channel  of  communication  between  the  true  aorta,  Ao,  and  the  left 
ventricle.  All,  Allantois.  Ao,  Aortic  division  of  the  aortic  limb  of  the  heart.  Au,  Right  auricle,  b,  See 
"<;."  c,  See"a."'  card',  Superior  part  of  the  cardinal'  vein  (the  anlage  of  the  azygos).  card",  Inferior  part  of 
the  posterior  cardinal  vein,  d.i,  Opening  of  the  first  gill-pouch  into  the  pharynx,  the  pharynx  being  indicated 
by  dotted  shading,  cl.2,  Opening  of  the  second  gill-pouch  into  the  pharynx,  cl.i,,  cl.4,  Entodermal  portions 
of  the  third  and  fourth  gill-clefts,  c.om,  Dotted  outline  of  the  omental  or  lesser  peritoneal  cavity,  d,  Left 
(hut  of  Cuvier.  D.C,  Right  duct  of  Cuvier,  the  main  venous  trunk  entering  the  heart  from  the  right  side. 
D.V,  Ductus  venosus  Arantii.  F.W,  Foramen  epiploicum  (of  Winslow),  drawn  in  black.  F-Pp,  Foramen, 
drawn  in  black,  between  the  pleural  and  peritoneal  cavities.  The  foramen  is  bounded  by  the  lung,  liver,  and 
Wolffian  body;  the  figure  shows  the  pleural  side  of  the  opening.  If  we  pass  through  the  foramen  as  drawn  we 
reach  the  abdominal  cavity.  The  outline  of  the  pleural  cavity  is  marked  by  a  dotted  line.  Gen,  Genital 
tubercle,  represented  as  somewhat  displaced  from  the  median  line,  which  it  really  occupies.  G.R,  Genital 
ridge.  Jug',  -/«.?",  Jugular  or  anterior  cardinal  vein.  Li,  Liver,  l.s,  Anlage  of  the  lateral  venous  sinus. 
»i.\;  Vein  of  the  inferior  maxilla  or  mandible.  P,  Pulmonary  division  of  the  aortic  limb  of  the  heart,  p.c, 
IVrii  urdial  cavity.  PI,  Dotted  outline  of  pleural  cavity.  Rec,  Rectum.  Sc,  Sub-cardinal  vein,  which  is 
derived  from  the  cardinal  and  on  the  right  side-  of  the  body  forms  part  of  the  vena  cava  inferior.  Scl,  Sub- 
c  lavian  vein,  six,  Anlage  of  the  superior  longitudinal  venous  sinus,  which  is  formed  by  the  union  of  th- 
veins,  /..?.,  from  the  sides  to  make  a  single  vessel  between  the  r(>r,.),r.,]  ; ...  '  .ier.  i'ni.if, 

Right   umbilical   vein      v,    x  Vena 

'>,    Wo! (Viar. 
of   I  lie   n'glil   and 


GENERAL  ANATOMY  OF  EMBRYO  OF  12  MM. 


241 


sis 


cL.i 


cl.2. 


W.D 


cl.5 


cU 


FIG.  17-0. 


1 6 


242 


STUDY  OF  PIG  EMBRYOS. 


243 


FIG.  178. — PIG  EMBRYO  OF  12.0  MM.     RECONSTRUCTION  FROM  THE  TRANSVERSE  SECTIONS.  Si- 
To  show  especially  the  cephalic  nerves,    c.i,  0.2,  c.$,  Cervical  nerves,    ch.ty,  Chorda  tympani.    curt,  < 
commissure   connecting  with   the   jugular  ganglion,   j.     Dien,  Diencephalon.     ex,  Eternal  ^ 
spinal  accessory  nerve.     F,  Froriep's  ganglion,  which  in  man  completely  disappe  : 
H,  Cerebral  hemisphere.     ;,  Jugular  ganglion,  from  which  the  small  avnicular  nerve  rv> 
the  glosso-pharyngeus.    L,  Lens,     l.r,  Laryngeal  branch  of  the  glcsso-pharyngeal  nerve, 
ficial  petrosal  nerve,     md,  Mandibular  branch  of  the  trigeminal.     Mesi'tt,  Mesencephalon.     .1 
cephalon.     mx,  Maxillary  branch  of  the  trigt-minal.     Myelen,  Myelencephalon.     n,  Xodo'-al  g:1.^ 
Nasal   pit.      Op,    Optic  cup.      oph,  Ophthalmic    branch    of    the    trigeminal.      Ot. 
ganglion,     ph.r,    Pharyngeal    branch    of    the  glosso-pharyngeal    n^rve.     rec.    Recurrent    laryngeal 
s,   Superior    ganglion,      s-l,  Semilunar  ganglion.     Telen,  IV'encepha'on.     ty,  Tympanic  nerve 
Roof  of  the  fourth  ventricle.     3  Oculomotor  nerve.     4,  Trochli  a  5,  Trigeminal  nerve.    6,  A! 

nerve.      7,    Geniculate   ganglion.     8,  Acoustic   ganglion.     9,  Glo  so-pharyngeal   nerve.     10.   \ 
ir,  Spinal  accessory  nerve.     12,  Hypoglossal   nerve.      Y,  20  diar  s. — (Drawn  by  F.   T.  Li 


244 


STUDY  OF  PIG  EMBRYOS. 


/. 


*    V 


FIG.  179. 


GENERAL  ANATOMY  OF  EMBRYO  OF  12  MM. 


245 


Oe,   Tra 


FIG.-  180. — PIG  EMBRYO  OF  12.0  MM.     RECONSTRUCTION  FROM  THE  TRANSVERSE  SECTIONS,  SERIES  5. 

The  embryo  has  been  drawn  as  if  transparent,  to  show  the  form  of  its  pharynx  and  the  relations  of  the  pharyngeal 
gill-pouches  to  the  grooves  on  the  outer  surface  of  the  embryo.  Cbl,  Cerebellum.  C.S,  Cervical  sinus.  Dien, 
Diencephalon.  ep.b,  Epibranchial  body.  H,  Cerebral  hemisphere.  Hy,  Hypophysis,  which  arises  as  an 
evagination  from  the  oral  cavity.  Inf,  Infundibular  gland,  which  arises  as  a  median  evagination  from  the 
floor  of  the  fore-brain,  l.gr,  Lachrymal  grooVe.  m,  Maxillary  process.  M.b,  Mid-brain.  Mdb,  Mandibular 
process.  Md.ob,  Medulla  oblongata.  na.ex,  External  nasal  opening;  the  nasal  cavity  is  dotted,  na.pl,  Nasal 
plate  separating  the  nasal  pit  from  the  oral  cavity;  by  the  rupture  of  this  plate  the  inlernal  nares  is  formed 
later,  ni,  Internal  nares.  nod,  Nodulus  thymicus.  Oe,  (Esophagus.  Op,  Eye.  Ot,  Otocyst.  Sp.c,  S]  .  >a! 
cord.  Th,  Median  thyroid  gland,  thym,  Anlage  of  the  thymus  (part  of  the  third  gill-pouch).  Tra, 
Trachea.  Yen.  IV,  Fourth  ventricle,  i,  Entodermal  pouch  of  the  first  branchial  cleft,  the  anlage  of 
the  Eustachian  tube  and  tympanum.  2,  Entodermal  pouch  of  the  second  gill-cleft;  it  actually  opens  to  the 
exterior.  3,  Entodermal  pouch  of  the  third  branchial  cleft.  4,  Entodermal  pouch  of  the  fourth  gill-cleft, 
the  lower  fork  of  which  is  the  anlage  of  the  lateral  thyroid.  X  20  diams. — (Drawn  byF.  T.Lewis.} 


246  STUDY  OF  PIG  EMBRYOS. 

Figure  178  shows  the  disposition  of  the  cephalic  and  upper  cervical  nerves 
and  also  the  position  of  the  nasal  cavity, '  the  eye,  and  the  otocyst. 

Figure  180  gives  an  outline  of  the  head  and  combines  an  indication  of  the 
external  modeling  of  the  gill-arches,  with  a  representation  of  the  shape  of  the 
pharynx. 

Pig  Embryo  of   6  mm.     Studied  in  Sections. 

Of  this  stage  three  transverse  sections  are  figured  in  order  to  give  more  exact 
notions  as  to  the  structure  of  neuromeres,  of  the  pharynx,  and  of  the  secondary 
segments. 

Transverse  Section  through  the  Fourth  Ventricle  (Fig.  181). — The  section  is  taken 
through  the  level  of  the  head,  and  may  be  directly  compared  with  figure  189. 
The  relations  are  so  closely  similar  that  it  is  unnecessary  to  describe  the  present 
section  (Fig.  181)  in  detail.  The  explanation  of  the  figure  is  sufficient  for  the 
identification  of  the  parts.  The  otocyst  is  large  and  conspicuous,  and  the  arrange- 
ment of  the  nerves  is  essentially  similar  to  what  we  find  in  the  older  embryos. 
The  neuromeres,  however,  are  very  distinct,  especially  those  upon  the  left  side 
of  the  embryo,  N.  i,  2,  3,  4.  Of  these,  the  third  is  perhaps  the  most  characteris- 
tic. Each  neuromere  is  separated  from  its  fellow  by  an  internal  sharp  ridge,  so 
that  the  inner  boundary  of  each  neuromere  toward  the  cavity  of  the  fourth  ventricle 
is  a  small  arc  of  a  circle.  The  cells  are  elongated  and  are  placed  radially  to  the 
inner  curved  surface  of  the  neuromere.  A  thin  but  distinct  layer  of  ectoglia  is 
present.  The  light  line,  which  marks  the  boundary  between  the  adjacent  neuromeres, 
is  produced  by  the  comparative  absence  ol  nuclei.  As  to  the  number  of  neuro- 
meres our  knowledge  is  still  defective;  nor  have  we  yet  succeeded  in  making  sure 
of  their  exact  relation  to  the  nerves  of  the  head,  though  such  a  relation  evidently 
exists.  Thus  we  find,  for  example,  that  the  facial  nerve  is  always  connected  with 
neuromere  2  of  our  figure,  and  the  glosso-pharyngeal  nerve  with  neuromere  4. 

Transverse  Section  through  the  Region  of  the  Branchial  Arches  (Fig.  182). — The 
branchial  arches  are  much  more  conspicuous  at  this  stage  than  in  later  ones,  being 
separated  from  one  another  by  deep  ectodermal  depressions,  figure  29,  I,  II,  III, 
IV;  and,  although  ///  and  IV  are  already  being  turned  in,  preparatory  to  the 
formation  of  the  cervical  sinus,  they  are  still  distinct  and  their  order  in  the  series 
is  evident.  The  section  (Fig.  182)  shows  on  the  dorsal  side  the  spinal  cord,  in 
which  we  can  already  recognize  the  subdivision  into  dorsal  zone,  D.Z,  and  ven- 
tral zone,  V.  Z.  To  the  dorsal  zone  is  appended  the  dorsal  root;  from  the  middle 
of  the  ventral  zone  comes  off  the  ventral  root  of  a  cervical  nerve,  N.  Just  between 
the  dorsal  root  and  the  wall  of  the  spinal  cord  can  be  seen  the  section  of  the  ac- 
cessory nerve.  The  secondary  somite,  My,  is  sharply  defined  and  has  a  distinct 
growing  edge  showing  at  its  upper  limit  in  the  section.  The  inner  leaf  of  the 
secondary  somite  is  stained  more  lightly  than  the  neighboring  tissue,  corresponding 
to  the  modifications  which  the  cells  are  undergoing  preparatory  to  their  change 


EMBRYO  OF  6  MM.  STUDIED  IN  SECTIONS. 


247 


into  young  muscle-fibers.  In  the  12  mm.  pig  in  this  region  the  cells  of  the  muscle- 
plate  have  already  broken  apart  and  no  distinct  plate  can  any  longer  be  recognized. 
Below  the  muscle-plate  follows  the  section  of  the  anterior  cardinal  vein,  Card. 
Lower  down  and  in  the  median  line  we  have  the  section  of  the  pharynx,  Ph, 


Ven.IV. 


Card 


A  .car- 


V  en. 111.     Md. 


FIG.  181. — PIG,  6.0  MM.     TRANSVERSE  SERIES  9,  SECTION  90. 

A.car,  Carotid  artery.  Card,  Card',  Anterior  cardinal  vein.  Md,  Medullary  wall  of  the  fore-brain.  A'.i,  X.2 
JV.3,  N.4,  Neuromeres  of  the  hind-brain.  A*. 5,  Trigeminal  ganglion.  N.-j,  8,  Acustico-facial  ganglion. 
N.g,  Root  of  the  glosso-pharyngeal  nerve.  Ot,  Otocyst.  Ven.III,  Third  ventricle  or  cavity  of  the  fore-brain. 
Ven.IV,  Fourth  ventricle.  X  35  diams. 

lined  by  the  epithelial  entoderm.  The  pharynx  is  surrounded  by  the  very  large 
aortic  vessels,  which  start  from  the  ventral  side  of  the  pharynx,  and  pass  upward 
along  its  sides  to  join  the  descending  aorta,  Ao.d.^,  at  about  the  level  of  the  jugu- 
lar veins.  The  vessels  shown  are  the  fourth  aortic  arches.  Their  symmetry  and 


248 


STUDY  OF  PIG  EMBRYOS. 


their  relations  to  the  pharynx  are  beautifully  demonstrated  in  this  section.  Below 
the  aorta  we  find  a  section  of  the  third  internal  gill-cleft,  cl.III,  a  narrow,  slit- 
like  cavity  lined  by  entoderm.  By  following  the  series  of  sections,  the  connection 
of  this  cavity  with  that  of  the  pharynx  can  be  traced,  thus  demonstrating  that  the 


Olf. 


F.B 


D.Z. 


Nch. 


Ill, IV. 


FIG.  182. — PIG,  6.0  MM.     TRANSVERSE  SERIES  9,  SECTION  171. 

A».d.4,  Descending  aorta  receiving  the  right  fourth  aortic  arch.  Card,  Anterior  cardinal  vein.  cl.III,  Third 
entode.rmal  gill-cleft.  D.Z,  Dorsal  zone  of  spinal  cord.  F.B,  Fore-brain.  Hy,  Hyoid  branchial  arch.  Mdb, 
Mandibular  branchial  arch.  My,  Muscle-plate.  N,  Nerve.  Nch,  Notochord.  Olf,  Olfactory  plate. 
Ph,  Pharynx.  V.Z,  Ventral  zone  of  spinal  cord.  I,  II,  III,  IV,  First  to  fourth  ectodermal  gill-clefts. 
X  35  diams. 

cleft  is  an  evagination  of  the  pharynx,  as  are  all  the  gill-clefts.  On  the  left-hand 
side  of  the  embryo  the  junction  of  the  entoderm  of  the  internal  pouch  with  the 
ectoderm  is  shown.  The  two  germ-layers  have  united  to  form  a  typical  closing 
plate.  Above  the  third  gill-cleft,  the  outline  of  the  embryo  shows  a  deep  depres- 


EMBRYO  OF  6  MM.  STUDIED  IN  SECTIONS. 


249 


Sp.c. 


A  .is 


Cu. 


V.R. 


Scler. 


sion,  ///,  IV,  which  is  due  to  the  commencing  formation  of  the  cervical  sinus. 
From  the  upper  end  of  this  depression  runs  upward  the  ectodermal  fourth  cleft,  and 
from  its  lower  part  extend^  downward  the  ectodermal  third  cleft.  Between  the 
third  and  fourth  clefts  the  external  surface  of  the  embryo  protrudes  somewhat.  This 
protuberance  corresponds  to  the  so-called  fourth  branchial  arch.  Between  the 
third  external  cleft  and  the  second,  //,  is 
a  still  greater  protuberance  on  the  outside 
of  the  embryo.  This  marks  the  third 
branchial  arch.  The  third  aortic  arches 
are  somewhat  imperfectly  shown,  but  the 
connection  of  the  left  third  arch  with  the 
central  aorta  appears.  Between  the  second 
and  first  external  clefts  we  have  the  second 
or  hyoid  branchial  arch,  Hy;  and,  similarly, 
between  the  first  or  auditory  cleft,  7,  and 
the  oral  fissure,  which  separates  the  head 
from  the  body  of  the  embryo,  we  have  the 
very  large  and  protuberant  mandibular  arch, 
Mdb.  The  head  '  of  the  embryo  is  com- 
pletely separated  in  this  section  from  the 
body.  It  shows  the  cavity  of  the  fore-brain, 
F.B,  bounded  by  the  ectoderm  of  the  med- 
ullary wall,  and  on  one  side  also  shows 
the  thickening  of  the  epidermis,  Olf,  which 
forms  the  olfactory  plate  or  plakode,  which 
is  to  become  the  lining  of  the  nasal  pit. 

Transverse  Section  of  the  Lower  End  of 
the  Embryo  (Fig.  183). — Our  third  section 
is  very  near,  the  end  of  the  series.  Owing 
to  the  curvature  of  the  posterior  end  of  the 

body  of  younger  embryos  (compare  Fig.  FIG.  183.— PIG,  6.0  MM.  TRANSVERSE  SERIES  9, 
165;  pig,  7.5  mm.),  sections  taken  in  the 
plane  which  we  call  transverse  strike  the 
lumbar  region  so  as  to  give  longitudinal  sec- 
tions of  the  spinal  cord  and  primitive  seg- 
ments. Figure  183,  therefore,  shows  the 

cavity  of  the  spinal  cord,  Sp.c,  cut  for  a  very  long  distance.  At  the  upper  and 
lower  ends  of  the  section,  the  dorsal  side  of  the  spinal  cord  is  cut,  and  accord- 
ingly we  see  at  these  levels  sections  of  the  ganglia,  G,  on  either  side  of  the 
spinal  cord.  In  the  middle  of  our  section  the  ventral  portion  of  the  spinal  cord 
is  cut,  and  here,  therefore,  the  ventral  roots,  V.R,  of  the  nerves  are  displayed. 
The  somites  are  clearly  marked  by  the  external  configuration  of  the  embryo,  the 


SECTION  519. 
A. is,  Intersegmental  artery.     Cu,  Cutis  plate.     EC, 
Ectoderm.     G,  Ganglion,     muse,  Muscle-plate. 
Scler,    Sclerqtome,    auct.      Sp.c',    Spinal    cord. 
V.R,  Ventral  nerve-root.      X  50  diams. 


250  '  STUDY  OF  PIG  EMBRYOS. 

ectoderm,  EC,  forming  an  arch  over  the  outside  of  each  segment.  Each  mesodermic 
somite  shows  three  distinct  parts:  next  to  the  ectoderm  the  broad,  epithelioid 
cutis  plate,  within  which  comes  the  spindle-shaped  section  of  the  inner  portion  of 
the  somite,  muse,  the  anlage  of  the  skeletal  muscles;  and,  third,  an  expanding 
mass  of  mesenchyma,  Scler,  which  is  sometimes  termed  the  sclerotome.  This  term, 
however,  is  not  wholly  felicitous,  because  this  mesenchyma  forms  not  only  the  seg- 
ments of  the  skeleton,  but  the  connective  tissue  of  the  whole  region  about  the  spi- 
nal cord  in  the  dorsal  part  of  the  embryo.  .  The  figure  shows  very  clearly  that  the 
ganglia  and  ventral  nerve-roots  are  arranged  in  exact  conformity  to  the  segments, 
and  it  can  be  easily  observed,  by  following  through  the  series  of  sections,  that  for 
each  somite  there  is  one  ganglion  and  one  ventral  root.  It  also  shows  that  the 
ventral  roots  reach  directly  to  the  muscle-plate.  The  muscle-plate  is  histologically 
partly  differentiated,  for  its  cells  have  already  elongated  in  a  direction  parallel 
with  the  longitudinal  axis  of  the  embryo,  and  their  nuclei  also  have  become  much 
larger  than  any  other  nuclei  in  the  neighboring  parts  of  the  embryo,  being  per- 
haps three  times  as  large  as  the  mesenchymal  nuclei  of  the  sclerotome.  They  are 
oval  in  form,  contain  many  fine,  and  usually  one  or  two  somewhat  larger  granules, 
the  larger  ones  being  deeply  stained;  but  the  nuclei,  as  a  whole,  are  stained  more 
lightly  than  their  neighbors.  Each  somite  is  very  clearly  separated  from  its 
neighbors,  and  between  the  ends  of  the  adjacent  muscle-plates  there  is  a  small 
clear  space  entirely  free  from  cells  and  extending  outward  to  the  epidermis.  Just 
inside  of  this  space  in  every  case  is  a  small  blood-vessel,  the  intersegmental  artery, 
A. is.  The  intersegmental  arteries  are  small  branches  which  arise  in  symmetrical 
pairs  from  the  dorsal  aorta. 

Pig  Embryo  of  9  mm.  Studied  in  Sections. 

Pig  embryos  of  this  stage  supplement  very  instructively  those  of  12  mm.  It 
will,  of  course,  be  advantageous  for  the  student  to  prepare  serial  sections  himself. 
When  that  is  not  possible,  there  should  at  least  be  sections  prepared  for  the  lab- 
oratory which  the  student  may.  examine.  Four  sections  are  illustrated  and  described 
below.  They  have  been  chosen  to  supplement  the  descriptions  of  the  sections  of 
the  pig  of  12  mm.,  and  they  will  be  found  to  illustrate  certain  fundamental  mor- 
phological relations  in  the  embryo  more  clearly  than  older  stages. 

Sagittal  Section  to  the  Right  of  the  Median  Plane  (Fig.  184). — In  the  accorri- 
panying  figure  184  the  cephalic  end  of  the  embryo  is  omitted;  a  portion  of  the 
heart,  the  entire  length  of  the  Wolffian  body,  and  the  tail  are  included.  The 
dorsal  outline  of  the  embryo  forms  a  characteristic  curve.  A  long  series  of  spinal 
ganglia,  G,  is  shown  arranged  in  regular  succession  and  following  the  curvature 
of  the  back.  The  ganglia  are  easily  recognizable  by  their  dark  staining;  each  of 
them  is  so  large  as  to  occupy  at  least  four  fifths  of  the  length  of  the  segment  to 
which  it  belongs.  The  boundaries  between  the  adjacent  primitive  segments  are 
indicated  by  the  positions  of  the  intersegmental  arteries,  A. is'.  Even  when  their 


EMBRYO  OF  9  MM.  STUDIED  IN  SECTIONS. 


251 


Ao.D.     Pul.     V.h.c.        V.s.      Au. 


A. is 


W.b. 


msth. 


Cce. 


V.msn 


FIG.  184. — PIG,  9.0  MM.     SAGITTAL  SERIES  53,  SECTION  213. 

A. is,  Intersegmental  artery.  All,  Allantois.  Ao,  Median  aorta.  Ao.D,  Descending  aorta.  Art.v,  Arteria 
vitellina.  Au,  Auricle.  Clo,  Cloaca.  Cue,  Ccelom.  Ent,  Entoderm.  G,  Ganglion.  G.b,  Gall-bladder. 
Glo,  Glomerulus.  Li,  Liver,  msth,  Mesothelium.  Pul,  Lung.  Seg,  Segment.  Som,  Somatopleure.  Sp.c, 
Spinal  cord.  Um.w',  Upper  wall  of  umbilicus.  Um.w",  Lower  wall  of  umbilicus.  Ve,  Vein  in  liver.  Yen, 
Ventricle  of  heart.  V.h.c,  Vena  hepatica  communis.  Vil,  Villus.  V.msn,  Vena  sub-cardinalis.  V.p,  Portal 
vein.  V.s,  Valvula  sinistra.  W.b',  W.b",  Wolffian  body.  X  22  diams. 


252 


STUDY  OF  PIG  EMBRYOS. 


cavities  do  not  show,  the  position  of  these  vessels  is  marked  by  the  darker  line 
of  tissue.  The  origin  of  one  of  these  intersegmental  vessels  from  the  dorsal  aorta, 
Ao,  is  indicated  in  the  lower  part  of  the  figure.  The  Wolffian  body,  W.b',  W.b", 
extends  from  the  level  of  the  lungs  and  liver  well  down  toward  the  pelvic  end 
of  the  embryo.  Its  ventral  limit  is  marked  by  the  body-cavity,  Ccc,  and  it  is, 
of  course,  covered  by  a  layer  of  mesothelium,  msth,  which  here,  as  everywhere  and 
at  all  stages,  forms  the  boundary  of  the  ccelom.  In  the  Wolffian  body  we  dis- 
tinguish readily  numerous  sections'  of  the  epithelial  Wolffian  tubules,  and  toward 
the  ventral  side  of  the  organ  the  characteristic  glomeruli,  Glo.  Between  the  glo- 
meruli  and  the  mesothelium  there  is  a  layer  of  mesenchyma,  but  between  the  tubules 
there  is  little  tissue,  the  intertubular  spaces  being  almost  entirely  occupied  by 
sinusoids  developed  from  the  cardinal  vein.  The  larger  sinusoid  or  venous  space, 
V.msn,  is  due  to  the  section  of  the  venous  trunk  which  joins  the  lower  end  of 
the  vena  cava  inferior,  and  is  known  as  the  sub-cardinal  vein.  In  the  upper  part 
of  the  figure  we  encounter  a  section  of  the  descending  aorta,  Ao.d,  and  of  the 
lungs,  Pul,  or  pulmonary  anlage.  The  latter  consists  of  a  ring  of  entoderm  bound- 
ing the  central  cavity  and  enclosed  by  a  thicker  layer  of  mesenchyma,  which,  again, 
is  bounded  by  a  layer  of  mesothelium.  The  space  or  ccelom  about  the  lung  is 
shown  in  the  figure  to  be  continuous  with  the  ccelom  of  the  abdominal  region. 
On  the  ventral  side  we  have  the  heart  partly  shown,  the  ventricle,  Ven,  being  so 
cut  as  to  exhibit  the  trabecular  structure  of  the  network  of  the  sinusoidal  spaces. 
The  auricle,  Au,  is  without  sinusoids.  The  great  venous  trunk,  vena  hepatica 
communis,  V.h.c,  opens  into  the  auricle,  the  opening  being  guarded  by  two 
valves,  that  on  the  dorsal  side  of  the  opening  in  the  figure,  V.s,  being  the  left 
valve.  The  vein  receives  blood  from  the  liver,  Li,  and  from  the  Wolffian  bodies, 
and  it  persists  in  the  adult  as  the  uppermost  part  of  the  vena  cava  inferior.  The 
duct-us  venosus  Arantii,  which  is  so  large  in  the  human  fetal  liver,  is  less  conspicu- 
ous in  the  pig;  the  ductus  is  the  venous  trunk  formed  by  the  union  of  the  portal 
and  umbilical  veins  within  the  liver;  it  joins  the  vena  cava  inferior  to  form  the 
vena  hepatica  communis.  The  liver,  Li,  consists  of  liver  cells  or  hepatic  cylinders 
and  numerous  sinusoids  of  many  diameters.  On  the  lower  side  of  the  liver  there 
is  a  considerable  accumulation  of  mesenchyma  by  which  the  liver  is  united  on 
the  one  end  to  the  body-wall,  Som,  to  the  umbilical  wall,  Um.w',  and  to  the, 
mesentery  by  which  the  intestine  is  suspended  from  the  liver.  In  this  mesen- 
chyma is  lodged  the  gall-bladder,  which  is  cut  thrice.  The  reference  line  G.b 
runs  to  the  uppermost  of  the  three  sections.  The  diameter  of  the  gall-bladder 
is  ^several  times  that  of  the  entodermal  intestine,  and  its  lining  epithelium  is  thicker 
than  any  other  epithelium  of  the  embryo  at  this  stage.  The  section  of  the  bladder 
nearest  the  portal  vein,  V.p,  corresponds  to  the  beginning  of  the  ductus  cysticu 
Underneath  the  liver  in  the  section  of  the  mesentery  is  situated  the  portal  vein, 
V.p.  From  the  mesentery  extends  out  the  intestine  (duodenum).  It  is  a  somewhat 
cylindrical  tube  which  curves  over  ventralward  and  passes  out  through  the  opening 


•  ' 


EMBRYO  OF  9  MM.  STUDIED  IN  SECTIONS.  253 

of  the  umbilicus.  It  consists  of  a  very  small  tube  of  entoderm,  Ent,  with  only  a 
small  internal  cavity  (compare  Fig.  186,  Reel.}.  The  thickness  of  the  intestinal 
wall  is  due  chiefly  to  the  considerable  development  of  the  mesenchyma.  The  ex- 
ternal covering  of  the  intestine  is  a  layer  of  mesothelium  which  becomes  the  peri- 
toneal epithelium  of  the  adult.  In  the  tissue  of  the  organ  we  distinguish  the 
narrow  vitelline  artery,  Art.v.  The  umbilical  opening  is  very  wide  and  is  bounded 
both  above  and  below  by  a  prolongation,  Um.w',  Um.w",  of  the  somatopleure 
of  the  embryo.  The  wall  on  the  upper  side  is  much  thicker  than  on  the  lower. 
The  umbilical  opening  is  partly  occupied  by  the  duodenum.  Appended  to 
the  inferior  wall  of  the  umbilicus  is  the  allantois,  All,  which  arises  from  the 
enlarged  caudal  end  (cloaca),  Clo,  of  the  intestine.  It  passes  out  first  inward, 
then  makes  an  acute  but  rounded  angle,  and  extends  outward  through  the  um- 
bilical opening.  It  may,  therefore,  be  said  to  consist  of  two  limbs,  one  within 
the  body  of  the  embryo  joining  the  cloaca,  and  the  other  passing  out  through 
the  umbilical  opening.  The  limb  arising  from  the  cloaca  is  completely  united 
with  the  body-wall,  and  is,  of  course,  upon  the  side  toward  the  ccelom  covered 
in  by  mesothelium.  The  lining  of  the  allantoic  cavity  is  an  epithelium,  and  is  a 
portion  of  the  entoderm.  Along  the  second  limb  of  the  allantois  the  mesothelium 
on  the  side  toward  the  cavity  of  the  umbilicus  forms  a  series  of  clumsy  projections, 
Vil,  the  mesothelial  ^illi  of  the  allantois.  They  are  smallest  toward  the  embryo 
and  increase  in  size  distally.  With  higher  power  one  can  see  that  the  mesothelium 
of  the  villi  is  very  thin  and  the  mesenchyma  in  their  interior  of  quite  loose  texture. 
In  later  stages  the  mesothelium  grows,  the  mesenchyma  in  large  part  disappears, 
and  the  villi  then  seem  hardly  more  than  small  bags  of  mesothelium  with  but 
little  contents,  save  some  coagulum.  They  continue  to  enlarge  until  the  embryo 
is  17  or  1 8  mm.  long,  after  which  they  begin  to  abort.  In  these  older  stages  the 
villi  extend  far  into  the  abdomen  and  are  packed  in  between  the  abdominal 
viscera,  presenting  curious  appearances  in  section.  As  the  tail  of  the  embryo  is 
bent  to  one  side,  it  offers  us  a  section  of  a  portion  of  the  spinal  cord,  Sp.c,  and 
at  its  tip  a  glimpse  of  three  primitive  somites,  Seg. 

Frontal  Section  through  the  Mid-brain  and  Fore-brain  (Fig.  185). — Comparison 
with  figure  165  (pig,  7.5  mm.)  will  make  it  clear  that  in  a  frontal  series 
obtain  a  few  sections  of  the  head  which  include  only  mid-brain  and  fore-brain  a>id 
show  no  other  special  cephalic  structures.  The  mid-brain,  M.B,  is  somewhat 
rounded  in  form  and  passes  over  into  the  fore-brain,  which  is  quite  long  and 
which  already  shows  traces  of  its  subdivision  into  two  parts,  the  diencephalon, 
Dien,  which  lies  nearest  to  the  mid-brain,  and  the  prosencephalon,  Pros,  which 
constitutes  the  terminal  portion  of  the  brain  and  which  produces  the  lateral  expan- 
sions which  are  to  form  the  cerebral  hemispheres.  The  expanding  prosencephalon 
is  separated  by  a  constriction  from  the  diencephalon,  which  in  its  turn  is  similarly 
separated  from  the  mid-brain.  The  diencephalon  and  prosencephalon  together  rep- 
resent the  fore-brain.  They  are  subdivisions  of  the  primary  first  cerebral  vesicle. 


254 


STUDY  OF  PIG  EMBRYOS. 


It  is  important  to  note  that  they  do  not  correspond  to  complete  subdivisions,  and 
have  not  the  same  morphological  value  as  the  three  primary  vesicles.  The  histo- 
logical  development  is  much  less  advanced  than  in  the  pig  of  12  mm.  The  ecto- 
derm is  very  thin,  consisting  for  the  most  part  of  a  single  layer,  of  cells,  but  here 
and  there  the  formation  of  a  second  layer  is  seen  to  be  beginning.  The  mesoderm 
is  very  simple  in  character  and  almost  uniform  in  appearance,  but  there  is  a  dis- 
tinct difference  between  the  mesenchyma  around  the  brain  and  that  underneath  the 
epidermis,  the  former  having  cells  farther  apart.  This  is  almost  the  first  stage  in 

the  differentiation  of  the  arachnoid  zone  around  the 

Md..  -  x""*^j«siw ,;  brain.     The  pia  mater,  however,  though  quite   thin, 

is  well  defined  by  the  condensation  of  the 
mesenchymaF  cells  and  by  the  somewhat  numerous 
small  blood-vessels  in  it.  The  medullary  wall  is 
everywhere  quite  thick  and  crowded  with  nuclei. 
In  the  region  of  the  diencephalon  the  ectoglia  is 

Dien.  J :. '^i-tliiB     ^^&:; ';•'.'•  .-0   1    distinctly   formed,   but   elsewhere    has  hardly  begun 

its  differentiation.  On  the  inside  of  the  medullary 
wall,  close  to  the  surface,  there  are  everywhere 
very  numerous  mitbtic  figures. 

Frontal  Section    through    the    Umbilical    Opening 
(Fig.    186). — The    illustration    is    part   of    the    same 
section  in  the  series  from  which  figure  185  is  taken. 
For    convenience    of    comparison    the    position    has 
been    reversed    so    as    to    bring   the  dorsal    side   of 
the   embryo   uppermost    in    figure    186.      It   results 
from    this   that  right   and   left '  sides  of  the  embryo 
FIG.    185.— PIG,    9.0    MM.     FRONTAL    are   reversed   in    the    engraving    as    compared    with 
SERIES  54,  SECTION  194.  the  other  figures  of  transverse  and  frontal  sections. 

Dien,  Diencephalon.    M.B,  Mid-brain.    By  examining   figure  166  (pig,   io  mm.)  the  student 

Md,  Medullary  wall  of  brain,     mes,         .., 

,,  D      T>  u  i        will  see  that  sections  in  the  frontal  plane,  owing  to 

Mesenchyma.   Pros,  Prosencephafon. 

"v,  Vein,    x  22  diams.  the  curvature  of  the  posterior  end- of  the  body- wall, 

furnish  transverse  sections  of  the  spinal  cord  of  the 

pelvic  region.  Therefore,  the  section  here  figured,  although  part  of  a  frontal  series, 
is  directly  comparable  to  a  transverse  section  of  the  body.  In  the  upper  part  of 
the  figure  we  have  the  spinal  cord,  Sp.c,  and  on  one  side  of  that  the  ganglion, 
G.  Owing  to  the  spiral  twist  of  the  embryo  the  section  is  not  symmetrical,  so  that 
the  posterior  limb,  P.L,  appears  only  on  one  side  of  the  section.  Laterad  from 
the  nerve  shown  in  the  figure  is  the  large  muscle-plate,  My,  the  cells  of  which  are 
already  beginning  to  change  into  muscle-fibers.  On  the  dorsal  side  of  the  plate  we 
find  its  growing  edge,  m.pl,  where  the  tissue  of  the  muscle-plate  proper  bends 
over  and  passes  continuously  into  the  external  wall  of  the  somite.  From  this 
growing  edge  the  cells  are  added  to  the  muscle-plate  by  which  it  extends  upward. 


Ve. 


Mes. 


Pros. 


EMBRYO  OF  9  MM.  STUDIED  IN  SECTIONS. 


255 


The  similar  edge  on  the  ventral  side  provides  for  the  extension  of  the  muscle-plate 
downward.  In  the  median  line,  below  the  spinal  cord  is  the  small  notochord,  Nch, 
and  the  large  median  dorsal  aorta,  Ao.  In  the  ventral  portion  of  the  embryo 
appears  the  large  body-cavity  into  which  protrude  the  Wolffian  bodies  and  the  intes- 


•tn.pl.     G.       Sp.c. 


V.U.S 


•  '•'i"ftv^ 


FIG.  186. — PIG,  9.0  MM.     FRONTAL  SERIES  54,  SECTION  194. 

Ao,  Aorta,  card,  Cardinal  vein.  F,  F.ctodermal  fold  at  the  border  of  the  limb-bud.  G,  Ganglion,  gen,  Genital 
ridge.  Glo,  Glomerulus.  In,  Small  intestine  (jejunum),  mes,  Splanchnic  mesoderm  (of  the  intestinal 
wall),  m.pl,  Dorsal  growing  edge  of  the  muscle-plate.  My,  Muscle-plate  of  secondary  segment.  Nch, 
Notochord.  P.L,  Posterior  limb.  Reel,  Large  intestine.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  V.U.D, 
Right  umbilical  vein.  V.U.S,  Left  umbilical  vein.  W.b,  Wolffian  body.  W.D,  Wolffian  duct.  X  35 
diams. 

tine.  The  ccelom  also  has  a  downward  prolongation  into  the  beginning  of  the 
umbilical  cord,  and  in  this  prolongation  lies  the  so-called  extra-embryonic  loop  of 
the  intestine,  In.  The  coelom  is  bounded  everywhere  by  the  layer  of  mesothelium 
represented  in  the  engraving  as  a  continuous  line.  With  a  higher  power  the  meso- 


256  STUDY  OF  PIG  EMBRYOS. 

thelium  is  seen  to  consist  of  a  single  layer  of  cells,  but  varying  somewhat  in 
thickness  in  different  regions.  By  following  the  contour  of  the  mesothelium  the 
student  will  recognize  at  once  that  all  of  the  viscera  are,  in  the  anatomical  sense, 
outside  of  the  ccelom.  The  Wolffian  bodies,  W.b,  are  voluminous  organs  pro- 
jecting from  below  the  aorta  on  either  side  of  the  large  intestine,  Reel,  and  extend- 
ing far  into  the  abdominal  cavity.  At  the  lower  ventral  edge  of  the  Wolman  body 
appears  the  Wolffian  duct,  W.D,  a  wide,  longitudinal  canal  into  which  the  Wolffi- 
an  tubules  open.  The  large  size  of  the  duct  is  characteristic  of  this  stage.  In 
later  stages  it  is  smaller.  The  tubules  are  very  large,  contorted  in  their  course, 
and  appear,  therefore,  variously  cut.  They  are  formed  by  a  cuboidal  epithelium 
and  are  provided  with  a  sinusoidal  circulation.  The  endothelium  of  the  blood 
spaces  can  generally  be  seen  fitting  closely  against  the  epithelium  of  the  tubules. 
Here  and  there,  however,  there  is  some  mesenchyma  between  the  blood  spaces  and 
the  walls  of  the  tubules.  On  the  median  side  of  the  Wolman  body  are  the 
glomeruli,  which  are  of  large  size,  and  similar  in  structure  to  the  glomerulus  of 
the  permanent  kidney,  though  differing  from  the  renal  glomeruli  in  their  propor- 
tions and  in  the  details  of  their  structure.  It  is  not  difficult  to  make  a  reconstruc- 
tion of  the  course  of  a  single  tubule  by  following  it  through  a  few  neighboring 
sections.  The  general  course  of  a  tubule  is  in  the  transverse  plane,  but  it  is 
much  contorted.  Each  tubule  begins  at  one  of  the  glomeruli,  with  which  it  is 
in  open  communication.  .  It  then  bends  so  as  to  make  a  somewhat  irregular 
S-shaped  figure,  and  finally  opens  into  the  Wolffian  duct.  After  leaving  the  glomeru- 
lus it  widens  somewhat,  but  before  it  joins  the  Wolffian  duct  it  again  diminishes  in 
diameter.  The  changes  in  diameter  are  gradual.  The  blood  spaces  or  sinusoids 
of  the  Wolffian  body  are  derived  from  the  posterior  cardinal  veins.  The  veins  and 
tubules,  wrien  the  latter  first  become  distinct,  lie  near  together.  As  development 
con  tin"  ^«  both  enlarge  and  encroach  upon  one  another's  territory;  hence  there  is 
an  intimate  intercrescence  of  the  blood-vessels  and  of  the  tubules,  resulting  in  the 
formation  of  sinusoids.  The  whole  of  the  Wolffian  body  might  from  one  point  of 
view,  therefore,  be  regarded  as  a  modification  of  the  cardinal  vein,  and  morphologi- 
cally all  of  the  blood  spaces  between  the  tubules  belong  to  that  vein.  There 
remain  typically  two  portions  of  the  cardinal  vein  which  are  more  or  less  open  and 
distinct.  The  one  on  the  dorsal  side  of  the  Wolffian  body,  card,  may  be  conve- 
niently regarded  as  representing  the  original  cardinal  vein.  The  other,  on  the  ventral 
side  of  the  Wolffian  body,  is  at  first  not  a  very  distinct  channel,  but  gradually 
becomes  more  and  more  so,  and  is  known  by  the  distinctive  name  of  sub-cardinal 
vein.  It  is  a  vessel  of  great  morphological  importance,  since  on  the  right  side  of 
the  embryo  it  acquires  a  connection  with  the  liver  which  renders  it  possible  for  the 
blood  of  the  right  sub-cardinal  vein  to  pass  through  the  blood  spaces  of  the  liver 
directly  to  the  heart.  This  makes  a  very  direct  channel,  a  more  direct  one  than 
existed  previously,  when  the  blood  from  the  sub-cardinal  came  to  join  that  of  the 
cardinal,  passing  up  to  the  common  cardinal  and  then  back  to  the  heart.  The  new 


EMBRYO  OF  9  MM.  STUDIED  IN  SECTIONS.  257 

channel  through  the  liver  rapidly  enlarges  and  becomes  recognizable  as  the  vena 
cava  inferior.  This  important  venous  trunk  is  a  combined  vessel,  comprising,  first, 
a  part  of  the  sinus  venosus  of  the  heart;  second,  the  vena  hepatica  communis;  third, 
a  large  channel  developed  from  the  sinusoids  of  the  liver;  fourth,  the  upper  part  of 
the  right  sub-cardinal  vein;  and,  fifth,  the  lower  part  of  the  right  cardinal.  The 
vena  cava  inferior  has  already  been  developed  in  the  pig  embryo  of  9  mm.  Be- 
tween the  two  Wolffian  bodies  hangs  down  the  large  intestine,  Reel,  suspended  by 
its  mesentery  in  the  median  line.  The  entodermal  portion  is  a  very  small  circle 
of  epithelium  with  an  extremely  minute  lumen,  which  in  the  section  is  scarcely 
larger  than  a  single  nucleus.  The  mesentery  and  intestine  are  covered  by  a  well- 
defined  mesothelium  and  have  a  considerable  amount  of  mesenchyma,  in  which 
there  is  no  distinct  histological  differentiation  beyond  the  presence  of  a  number 
of  small  blood-vessels.  At  this  stage  the  large  intestine  runs  nearly  in  the  median 
plane  to  the  pelvic  end  of  the  body.  In  the  opposite  direction,  toward  the  head, 
it  bends  to  the  left  of  the  embryo,  making  a  loop  which  passes  over  into  the  end  of 
the1  ileum.  The  ileum  forms  the  continuation  of  the  loop  and  extends  into  the 
ccelom  at  the  bas'e  of  the  umbilical  cord.  There  it  bends  back  and  returns  toward 
the  dorsal  side  of  the  embryo  to  pass  over  into  the  duodenum  and  join  the  stomach. 
Owing  to  the  fact  that  the  small  intestine  extends  into  the  extra-embryonic  ccelom 
of  the  umbilical  cord,  there  makes  a  loop,  and  returns  to  the  embryonic  region, 
we  get  typically  a  double  section  of  the  intestine  as  shown  in  the  figure,  one  of 
each  limb  of  the  loop.  The  entoderm,  In,  in  these  loops  forms  ^  small  ring,  which, 
however,  is  much  larger  than  the  entodermal  ring  of  the  la.  ui.c  at  this 

stage.     Each  loop  contains  a  large  amount  of  mesenchyma,  mes.  th< 
are  somewhat  crowded,  so  that  the  tissue  appears  dark  in  the  stained*  section, 
boundary  between  the  body  of  the  embryo  and  the  tissue  of   i! 
marked   by  the    position  of   the    two  umbilical  veins,  that   of    the    lett  : 
being  very  much    larger    than    that    of  the    right    side,    V.U.D.     By  following   do., 
the  somatopleure,  Som,  of  the  embryo,  it  will  be  seen  that  these  veins  are  lodged 
therein,  and    that   the   continuation    of   the   somatopleure  beyond   these   veins   forms 
the    substance    of     the    umbilical    cord.     The    limb-bud,    P.L,    is    a    large    mass    of 
rather  dense  mesenchyma,  entirely  without  muscles  or  nerves  and  covered  by  ecto- 
derm.    At   the   edge   of  the  limb-bud   the   ectoderm  shows  a  special   thickening,  F. 
The    theory  has   been    advanced    that   this    thickening   is    homologous  with    the  ecto- 
dermal    fold -which   produces  the  fin  of   fishes,  or  at  least  that  portion  of    the  fin  in 
which  the  fin-rays   are  developed. 

Frontal    Section    through    the   Second   and    Third    Gill-Clefts 
preparation    the    section    hits    the    posterior   wall,    Ot,    of    t: 
anterior     to    the    origin    of    the    glosso-pharyngeal     nerve.     The    appe; 
section   ol    the  hind-br?in   is    characteristic   for  this  region  of    young  €n 
deck-plate     has    grown     gradually     in    size    and     forms     a    wide     n 
the    ependyrhal     roc;  !ie    fourth     ventricle.     Owing     to    this 


258  STUDY  OF  PIG  EMBRYOS. 

deck-plate,  the  upper  or  dorsal  limits  of  the  dorsal  zones,  D.Z,  are  brought 
far  apart  and  the  cavity  of  the  hind-brain  is  thus  enlarged.  The  dorsal  zone  is 
divided  by  an  angle  in  the  interior  and  by  the  point  of  entrance  of  the  nerve-roots 
on  the  exterior  from  the  ventral  zone,  V.Z.  On  their  dorsal  side  the  dorsal  zones 
thin  out  and  pass  over  gradually  into  the  ependyma.  The  ependyma  consists  of  a 
single  layer  of  cells.  In  the  dorsal  zone  the  differentiation  of  the  three  primary 
layers  of  the  medullary  wall  has  scarcely  begun,  but  in  the  ventral  zones  the  three 
layers  are  already  distinguishable,  though  not  far  advanced  in  their  differentiation. 

EC.  mes.     Epen.  >. 


A. has. 


FIG.  187. — PIG,  9.0  MM.    FRONTAL  SERIES  54,  SECTION  459. 

/  .has,  Basilar  artery.  Ao.d,  Descending  aorta  of  the  left  side.  Ao.^,7 Third  aortic  arch.  Ao.^,  Fourth  aortic 
arch  arising  from  the  median  ventral  aorta.  Card,  Anterior  cardinal  vein.  cl.II.ex,  External  portion  of  the 
second  gill-cleft.  cl.III,  Third  gill-cleft.  D.Z,  Dorsal  zone  of  the  medulla  oblongata.  EC,  Ectoderm . 
Epen,  Ependymal  roof  of  the  hind-brain.  Hy,  Hyoid  arch,  mes,  Mesenchyma.  Ot,  Posterior  wall  of  the 
otocyst.  P.Ao,  Pulmonary  aorta.  Ph,  Pharynx.  Som,  Somatopleure.  V.Z,  Ventral  zone.  X  22  diams. 

In  the  floor-plate  there  are  two  layers.  Below  the  medullary  tube  lies  the  ba"silar 
artery,  14.. has,  and  below  that,  not  far  from  the  upper  wall  of  the  pharynx,  lies 
the  small  round  notochord  in  the  midst  of  loose  mesenchymal  cells,  which  have 
not  yet  begun  to  condense  themselves  about  the  notochord.  The  pharynx  is  a  wide 
space  of  rather  stnall  dorso- ventral  diameter,  and  having  a  much  thinner  layer  of 
v.KK1.,  ,11  on  its  dorsal  than  on  its  ventral  side.  Above  the  pharynx  on  either 
side  lies  the  section  of  the  descending  aorta,  Ao.d.  The  reference  line  to  this 
vessel  crosses  a  dark  mass  of  cells  which  belong  to  the  ganglion  nodosum  of.  4  he 
tenth  nerve.  Below  the  pharynx  the  section  shows  the  third  aortio  arch,  AO.T,, 
and  the  fourth  aortic  arch,  Ao.^,  just  springing  off  from  the  median  aortic  /trunk 
above  the  heart,  so  that  the  two  fourth  arches  are  connected  across  the  tfiedian 


EMBRYO  OF  12  MM.  STUDIED  IN  SECTIONS. 

'He.      Between   the   third  and   fourth  aortic  arches  on   either  side  is  a   small   cavity 

med    by   entoderm,    cl.III,    a    diverticulum    from    the    third    gill-cleft.     Immediately 

>elow    the    otocyst   is    the    anterior   cardinal    vein,    Card.     From    a    point    below    the 

ardinal    there    extends    a   prolongation,    Hy,   which   may  be   taken   as   a   portion   of 

the  hyoid  or  second  branchial  arch.     It  extends  downward     and  consists  of  a  mass 

of    mescnchyma    covered    by    ectoderm.     It    encloses    a  space,  cl.II.ex,   which  may 

be    regarded    as    the    external    portion    of    the    second    gill-cleft.     In    a    neighboring 

;on    (455)  the    prolongation    of    the    pharynx    shown  in  figure   187   can  be  traced 

farther     until     it    opens    into   this    space,    cl.II.ex.     The   second   cleft   is   open 

upon    both   sides  of    the    embryo,  the   first   and   third   have   closing   membranes,  the 

fourth  cleft  is  not  yet  so  far  developed  that  its  entoderm  has  come  in  contact  with 

the    epidermis    of    the    embryo.     The    second    cleft    probably    always    becomes    open, 

differing  in  this  respect  from  all  the  others.     Why  it  has  this  peculiarity  we  do  not 

know.     The    opening    does    not    persist,  but    the    exact    history  of    its    closure    is    at 

present   unknown.     The  process,   Hy,   described  as  shutting  in  the  external   p  >rtion 

«    the  second  gill-cleft  has  sometimes  been  termed  the  operculum,  because  it  covers 

a    gill-cleft  opening,   as  does  the  operculum  of    a  bony  fish.     In  the  lower  part   of 

our   figure    a    portion   of    the    somatopleure,  Som,   is    shown   where    it    extends   ven- 

tralward  to  form  the  wall   of  the  pericardial  cavity.     There  is  also  included  in  the 

drawing  a  part  of  the  pulmonary  aorta,  P.Ao. 

Pig  Embryo  of  12  mm.  Studied  in  Sections. 

A   pig   embryo  of  12  mm.  has  been  selected  as  the  center  of  study  in  this 
because    its    anatomical    relations    are    such    that    they    may   be   readily    grasped   by 
the    student   who    has    already   studied  'the  'anatomy    of   an   adult   mammal,    human 
or   other.     At   the   same   time   the   development   of   the   organs   is   so   advanced   that 
their   fundamental   relations   may   be   observed.     From   an   embryo   of   this   sill 
transition  to  the  study  of  younger  embryos  is,  even  for  the  beginner,  comp.i 
easy.     It  is  not  necessary  that  the  embryo  should  be  of  this  exact  size;   ind(   cl. 
may   be   somewhat   advantageous   for   the   student   to   have  an   embryo   a   millirt 
larger,  or  one,  two,  or  even  three  millimeters  smaller,  since  the  figures  and  < 
tions  referring  to  the    12   mm.   stage  will  enable   him  to  identify  all  the  striH»tttB 
to  be  found  and  yet  call  upon  him  for  the  exercise  of  care  and  judgment  in  ide 
fying,   from  the  data  given  in  the  following  pages,  the  various  parts  in  the  q 
what  different  stage  he  may  be  studying.     Of  12  mm.  pigs  the  author  has  ha<| 
his  disposal  five  good  series  belonging  to  the  Harvard  Embryological  Collect^ 

The    transverse    series   is   the    most   important,    and   should   form    the   bJ| 
the   study,   and   accordingly  most   of   the   sections   figured    are   from   su 
Next  in  importance  comes  the  sagittal  series,  but  it  is  desirable  that  e 
should  have  a  series  in  the  three  standard  planes  at  his  disposal  for  *tn'|; 
practical    laboratory   study   each   student   should   be   required    to  make    a 
accurate    camera    lucida   ^drawings    of    carefully    selected 


260 


STUDY  OF  PIG  EMBRYOS. 


name  correctly  all  the  parts  shown  in  each  section  and  to  identify  the  distribution 
of  the  three  germ-layers  in  every  case.*  A  sufficient  number  of  high-power  draw- 
ings ought  to  be  added  to  illustrate  the  character  of  the  various  tissues. 


284-       340  380 


572 


Fu;.  188. — RECONSTRUCTION  or  A  PIG  EMBRYO  OF  12.0  MM.  WITH  INDICATIONS  OF  THK  PLANES  OF -SECTIONS 

FIGURED. 
an,  Cloacal  membrane.     Ao,  Aorta.     Au,  Auricle.     A.um,  Umbilical  artery,     a.v,  Vertebral  artery,     bas,  Basilar 

artery,     c,  Anlage  of  caecum,     car,  Internal  carotid,     can.  Caudal  artery.     C.Ex,  External  carotid  artery. 

f.b,  Fore-brain.     G,  Spinal  ganglion,     g,   Gall-bladder,     h.b.  Hind-brain.     In,  Intestine.    Li,  Liver,     m.b, 

Mid-brain.     o/>,  Optic  .vesicle.     Ot,  Otocyst.     pan,  Pancreas.     Sf>,  Spinal  cord.     St,  Stomach.     Urn,  Umbilical 

opening.      Yen,  Ventricle  of  heart.     Ill,  I V,  V,  Aortic  arches. 


284  Frontal  section,  Fig.  203 

340  Frontal  section,  Fig.  204 

380  Frontal  section,  Fig.  205 

423  Frontal  section,  Fig.  206 

572  Frontal  section,  Fig.  207 


185  Transverse  section,  Fig.  189 

198  Transverse  section,  Fig.  191 

249  Transverse  section,  Fig.  192 

292  Transverse  section,  Fig.  193 

321  Transverse  section,  Fig.  194 

jx,  Transverse  section,  Fig.  105 

470  Transverse  section,  Fig.  196 

513  Transverse  section,  Fig.  107 

f\,j,  Transverse  section,  Fig.  198 

*K>r  making  camera  lucida  drawings,  a  i-inch  objective  will  be  found  convenient. 
is  recommended.     Compare  vhe  directions  for  drawing  in  Chapter  VIII. 


An  Abbe  camera 


TK.*  \S\  ERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  261 

The  accompanying  *f.«ure  188  represents  the  outline  of  the  pig  embryo  which 
was  cut  into  the  series  of  transverse  sections  from  which  figures  189  to  198  have 
been  made.  The  student  can  easily  identify  the  parts  in  the  figure  by  comparison 
with  that  of  the  pig  of  10  mm.  (Fig.  j66),  aided  by  the  accompanying  description 
of  the  same.  The  sections  of  this  embryo  are  lo/n  in  thickness,  and  are  966 
in  number,  not  1200,  as  the  student  might  expect.  The  discrepancy  is  due 
to  the  shrinkage  of  the  embryo  when  imbedded  in  paraffin.  The  shrinkage  is 
always  very  great,  and  in  the  case  of  embryos  causes  a  loss  of  almost  20  per  cent 
in  the  length;  but  as  it  seems  to  take  place  uniformly  throughout  the  embryo,  it 
causes  no  distortion,  so  that  the  embryo  in  paraffin  is  an  exact  though  greatly 
reduced  copy — so  to  speak — of  the  living  embryo.  It  should  be  remembered  that 
no  correct  measurements  of  the  size  of  organs  or  cells  can  be  obtained  from  sec- 
tions made  by  the  paraffin  method.  This  limitation  upon  the  use  of  sections 
too  often  forgotten.  The  horizontal  lines  indicate  approximately  the  levels  at  whic 
the  sections  here7  figured  belong.  For  convenience  the  direction  and  position  of  the 
frontal  sections  represented  in  figures  203  to  207  are  also  indicated  approximately 
on  the  same  outline,  although,  of  course,  the  frontal  series  was  from  another 
embryo. 

Pig  Embryo  of  12.0  mm.     Study  of  Transverse  Sections. 

The  figures  and  descriptions  here  presented  of  ten  sections  have  been 
selected  as  illustrating  the  most  important  structures,  with  the  exception  of  the 
umbilical  opening  and  of  the  kidney,  which  can  be  better  represented  in  sections 
from  older  or  younger  stages. 

Section  through  the   I  *  pp> 

by  the  line   185,  this  sectiQffcJ  ^  taken  from  a  level  about 

and  the  apex  of  the  otocyst,  Ot.  It  passes,  therefore,  through  the  fore-brain, 
and  the  fourth  ventricle,  Vcn.IV,  or  cavity  of  the  hind-brain.  The  section^H 
bounded  by  a  thin  layer  of  cpjfiermis,  between  which  and  the  brain-wall  there  is 
a  large  amount  of  mesenc  hymal  tissue.  Alongside  the  hind-brain  lies  a  series  of 
important  structures  imbe  \<\£d  in  the  mesenchyma,  which  are  identical  upon  the 

ciH«<i.    althoi.  Bter   somewhat'  in    the   section,    as   the   plane   of   cutting 

was    not    symm<  rse    for   the    head.     These    structures    are   in   the    fol- 

lowing   order:  .V.;  rigeminal     ganglior. ,     AT./,8,    the    acustico-facial    ganglion 

complex;  Ot,  the  otocyst,  an  oval  vesicle  with  very  distinct  epithelial  walls.  Next 
the  ninth  or  glosso-pharyngeal  nerve  (scarcely  appearing  in  the  section  on  the  right 
side  of  the  embryo  i  is  shown  by  the  upper  ^portion  of  its  ganglion  on  the  ' 
(the  right  in  tv  vi;  tb-» 

close    to    the   me'*' 
posteri<  « 
thr 


262 


STUDY  OF  PIG  EMBRYOS. 


to  the  ganglionic  commissure  which  extends  above  the  ^"'gin  of  the  hypoglossal 
nerve.  On  the  left  the  continuity  of  this  commissure  is  better  shown  than  on  the 
right,  where  it  offers  two  parts,  one,  com,  entirely  ganglionic,  and  another,  com.II, 
which  comprises  both  the  ganglionic  portion  and  fibers  which  share  in  the  forma- 


com.II '. 


JV.io. 


D.E. 


N.7,8. 


Ven.IV 


F.b. 


FIG.  189. — PIG,  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  185. 

com,  Ganglionic  commissure  between  hypoglossal  and  vagus  nerves.     com.II,  Ganglionic  commissure  with  libers 
of  the  root  of  the  spinal  accessory  nerve.     D.E,  Ductus  endolymphaticus.     F.b,  fore-brain.     Md,  Medulla 

1 ta.     N.f,,  Trigominal  ganglion.     N.-j,&,  Acustico-facial  ganglion.     N.io,   Jugular  ganglion  of  the 

' >tocvst.     Ven.IV,  Fourth  von*--'--'-       ^'  ^2  diams. 

•TT  large,   some- 
"    hind- 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  263 

marks  in  the  study  of -the  topography  of  the  embryonic  head.  The  nerve-cells  of 
the  ganglion  are  grouped,  for  the  most  part,  on  the  side  toward  the  ectoderm, 
where  they  are  closely  crowded  together,  making  a  deeply  staining  mass.  Nearer 
the  brain-wall  the  tissue  of  the  ganglion  is  much  less  condensed,  is  somewhat 
penetrated  by  small  blood-vessels,  and  contains  a  considerable  number  of  nerve- 
fibers,  which  are  gathered  into  small  bundles.  Toward  the  brain-wall  the  bundles 
become  distinct,  and  on  the  right  side  of  the  embryo  the  passage  of  the  nerve- 
fibers  into  the  brain  can  be  readily  seen.  The  nerve-fibers  -at  this  stage  are 
merely  neuraxons;  that  is  to  say,  thread-like  prolongations  of  the  bodies  of  the 
nerve-cells  (neurones).  The  fibers  are  entirely  without  sheaths.  They  stain  very 
lightly,  and  hence,  in  the  preparation,  may  be  detected  by  their  light  appearance. 
The  nerve-fibers  may  be  conveniently  rendered  conspicuous  by  counterstaining  the 
sections  with  Lyons  blue.  The  nerve-fibers  of  the  trigeminus,  which  enter  the  wall 
of  the  hind-brain,  form  in  part  a  bundle  of  fibers,  which  extends  along  past  the 
acustico-facial  ganglia  within  the  medullary  wall.  These  fibers  represent  the  com- 
mencement of  the  ascending  trigeminal  tract  of  anatomists.  The  other  ganglia 
associated  with  the  hind-brain  are  not  well  shown  in  this  section.  The  otocyst 
(compare  Fig.  42,  p.  79)  has  a  very  sharply  defined  epithelial  wall  and  is  im- 
bedded in  loose  mesenchymal  tissue.  On  the.  right  side  of  the  embryo  we  have 
the  ductus  .endolymphaticus,  D.E,  the  opening  of  which  into  the  .main  cavity  of 
the  otocyst  is  shown  on  the  left  side.  The  epithelial  wall  of  the  ductus  is  thicker 
than  that  of  the  greater  part  of  the  otocyst  proper.  The  wall,  Md,.,  of  the  hind 
brain  exhibits  already  characteristic  differentiations^  for  it  shows  clearly  the 
primitive  laprs;  the  outer  neuroglia  layer  (ectoglia^is  thin, -and  appears  light  in 
the  sectii-  *  :;  use  it  takes  the  stain  slightly.  It  is  in  this  outer  neuroglia  layer 
(ectoglia}  that  the  entire  sensory  nerve-fibers  are  p  ;.rnriiy  distributed,  and.  there- 
fore, it  is  in  a  po,  rmd  l'->  ract 
situated.  Next  to  the  ectoglia  comes  the  middle  layer,  in  \vbi 
the  medullary  wall  are  situated,  and  which  is,  therefore,  termed  the  neurone  <  > 
gray  layer  (cinerea),  easily  recognizable  under  the  microscope  by  its  brighter  color, 
which  is  due  principally  to  the  fact  that  the  nuclei  in  this  layer,  though  numerous, 
are  much  less  crowded  than  in  the  innermost  of  the  three  layers,  or  primitive 
ependymal  layer,  which  at  this  stage  is  quite  thick.  Owing  to  the  presence  of 
nuclei,  the  gray  layer  is,  of  course,  stained  much  more  than  the  ectoglia.  The  nu- 
clei of  the  brain- wall  shoty  as  yet  very  little  differentiation.  There  are  numerous 
mitotic  figures  which  are  situated  exclusively  clpse  to  the  inner  surface  of  the  brain- 
wall  in  the  fore-brain.  The  structure  «•,*'  the  for^-brain  is  similar,  but  the  develo* 
ment  is  less  advanced;  the  differentiation,  T)i~4ki  neurone  layer  is  only  v* 
ning,  and  it  has  acquired  little  thickness.  In  tho'^'Vl^bjain  we  ><> 
along  the  region  between  the  otocysts,  a  se* 
a  scalloped  outline  to  the  wall.  A  ^ 
nex  ^  one  of  the  spac<- 


204  STUDY  OF  PIG  EMBRYOS. 

a  neuromere.  The  neuromeres  correspond  in  number  and  position  to  the  neighbor- 
ing primitive  segments,  and  are,  therefore,  to  be  designated  as  segmental  structures.  . 
They  also  bear  an  evident  relation  to  the  development  of  the  nerves,  and  the  J 
V  accepted  hypothesis  is  that  from  each  neuromere  springs  a  single  nerve.  The  at- 
tempts which  have  been  made  to  verify  this  hypothesis  have  met  with  very  serious 
difficulties,  for  the  relations  are  extremely  complicated,  and  until  the  matter  shall 
have  been  much  more  thoroughly  investigated  than  at  present,  we  must  remain 
in  the  dark"  as  to  the  precise  morphological  value  of  the  neuromeres.  But,  inas- 
much as  they  appear  with  the  greatest  constancy  in  the  embryos  of  all  vertebrates, 
we  cannot  help  accepting  the  view  that  they  are  really  structures  of  fundamental 
importance.  At  the  stage  we  are  studying  the  neuromeres  have  already  begun  to 
lose  their  distinctness,  and  in  slightly  older  pigs  can  be  traced  only  with  difficulty. 
In  younger  stages  their  primitive  characteristics  are  better  shown  (compare  page 
246).  As  regards  the' blood-vessels  in  the  present  section:  there  are  small  branches 
of  the  veins,  which  show  outside  of  the  ganglionic  commissure,  com;  parts  of  the 
cardinal  vein  appear  in  close  proximity  to  the  trigeminal  ganglion,  and  again  at 
the  side  of  the  head.  In  the  median  line  between  the  fore-brain  and  hind-brain, 
or  nearer  to  the  layer,  appears  a  section  of  the  basilar  artery.  Near  the  fore-brain 
on  either  side  is  the  loop  of  the  carotid  artery.  There  are  several  important  points 
to  be  observed  in  the  region  between  the  trigeminal  ganglia  and  the  fore-brain.  In 
order  to  show  these  more  clearly,  a  separate  illustration  (Fig.  190),  on  a  larger 
scnle,  of  this  portion  of  the  section  is  given.  The  trigeminal  ganglion,  the  wall  of 
the  foio-brain,  and  the  wall  of  the  hind-brain  will  be  at  once  identified,  so  that  the 
correspondence  with  the  general  figure  is  easily  followed.  Between  the  trigeminal 
ganglion  and  the  fore-brain  are  four  veins,  two  of .  which,  Card'  and  Card'" ',  are 
larger  and  are  parts  of  the  main  cardinal  stem  passing  from  the  region  of  the 
hind-brain ^to  thai- of  the,  fore-brain,  while  the  ;-,  u  .. mailer  ones,  Card",  are  merely 
nches  of  the  same  vessel.  Close  to  the  section,  Card'",  of  the  cardinal  nearest 
the  fore-brain  lie  the  very  small  sections  of  the  fourth,  N.4,  and  third,  N.T,,  cere- 
bral nerves.  The  fourth  nerve  is  minute  in  size  and  lies  just  behind  the  vein. 
The  third  nerve,  thougfi  somewhat  larger,  is  also  very  small  and  lies  anterior  to 
the  vein  somewhat  on  its  medial  side.  Both  of  these  nerves,  owing  to  their 
small  dimensions,  are  somewhat  difficult  to  observe  with  the  low  power.  The  de- 
tailed figure  brings  out  more  clear/  other  points.  It  shows  very  clearly  the  junc- 
tion of  the  trigeminal.  A  -  , -tico-facial,  N.j,8,  ganglia  with  the  wall  of  the 
1  brain,  and  also  the  div^  'of  that  wall  into  i'  three  primary  layers,  the 
'  "/,  the  gray  layer  ..rid  .iie  inner  or  ependym  layer,  ^Epen,  and 
floor-plate  '  •:,  /<7r/>/?.  Immediately  below  it  is  the  basilar 
'n  is  the  s<  I  loop  of  the  carotid, 
"nd  oi  thv  basilar  artery,  which 
<y  symmetric" '  ' 
spond 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM. 


265 


designated  in  the  adult  as  the  posterior  communicating  branch,  by  which  the  end 
of  the  carotid  proper  anastomoses  with  the  basilar  artery.  At  the  side  of  the  fore- 
brain  appears  a  blood-vessel,  Card.L,  which  might  be  called  the  lateral  cardinal. 
It  is  a  branch  of  the  main  cardinal  stem  and  passes  over  the  side  of  the  fore-brain 


Ec.gl.     AT.  7, 8.     Cin.  Epen.        Raph.     Ven.IV. 


A.bas. 


Pia. 


A. car. 


Ven.III. 


Card.L. 


Arach. 

FIG.  190. — PORTION  OF  FIGURE  189  MORE  HIGHLY  MAGNIFIED. 

A.bas,  Basilar  artery.  A. car,  Internal  carotid  artery.  Arach,  Arachnoid  zone.  Cin,  Neurone  layer  of  medulla. 
Cut,  Cutis  layer.  Card',  Card",  Card'",  Anterior  cardinal  vein.  Card.L,  Lateral  branch  of  the  cardinal  vein. 
Ec.gl,  Ectoglia.  Epen,  Ependymal  layer.  G.tri,  Trigeminal  ganglion.  N.-$,  Oculo-motor  nerve.  N.4, 
Trochlear  nerve.  N.$,  Sensory  root  of  trigeminus.  N."j,8,  Acustico-facial  ganglion.  Pia,  Pia  mater. 
Raph,  Raphe  of  the  medulla  oblongata.  Ven.III,  Third  ventricle -of  the  brain.  Ven.IV,  Fourth  ventricle 
of  the  brain.  X  50  diams. 

toward    the    median   dorsal    surface    thereof,    where  it    meets   the   correspondir  - 
of   the    opposite   side,    with    \vhich    it    then    unites   to    form   a    siagle    me(' 
This    vessel    ultimately    acquires    great    siz     "^^   is    known    as    tht   m--x 


dinal  sinus.     Tt  is,  V>\vn   in  figure  17?. 
in   the   median   'ine,   pc  -ist  to   form 


of  the  vessels 
of   the   ;'  ' 


266  STUDY  OF  PIG  EMBRYOS. 

in  the  embryo  are  all  small  branches  of  the  veins  when  they  first  appear.  Their 
great  enlargement  does  not  occur  until  comparatively  advanced  stages.  Finally! 
attention  should  be  paid  to  the  following  important  modifications  in  the  mesenchyma. 
Already  there  has  been  a  rich  development  of  a  plexus  of  fine  blood-vessels  over 
the  surface  of  both  the  fore-  and  hind-brain  which  has  been  accompanied  by  a 
slight  condensation  of  the  mesenchyma  between  the  blood-vessels,  thus  markirfg  a 
distinct  membrane,  in  which  we  can  easily  recognize  the  pia  mater,  Pia.  Outside 
of  the  pia  mater  comes  a  relatively  broad  zone,  Arach,  in  which  the  cells  are 
widely  separated  from  one  another  and  are  connected  by  slender  and  long  processes, 
so  that  the  intercellular  spaces  are  very  extensive.  This  broad  zone  is  the'  anlage  of 
the  arachnoid  membrane.  It  is  much  more  differentiated  around  the  ventral  portion 
of  the  brain  than  around  the  dorsal  side.  Between  the  arachnoid  zone  and  the 
external  epidermis  the  mesenchyma  is  somewhat  more  condensed  and  the  cells  are 
elongated  in  form,  in  part  almost  spindle-shaped,  forming  a  layer,  Cut,  which  we 
may  consider  the  anlage  of  the  cutis,  and  perhaps,  also,  of  the  subcutaneous  tissue, 
but  this  is  doubtful.  Between  the  arachnoid  zone  and  the  cutis  zone,  so  placed 
that  they  cannot  be  quite  said  to  belong  to  either  one  or  the  other,  appear  numer- 
ous blood-vessels.  These  form  a  more  or  less  distinct  vascular  layer,  which  ap- 
pears with  remarkable  constancy  in  all  classes  of  vertebrates,  and  over  a  large 
part  of  the  body.  It  may,  therefore,  be  called  the  panchoroid.  It  is  unquestion- 
ably of  very  great  morphological  importance,  but  its  history  is  imperfectly  known. 

As  regards  the  histological  condition  of  the  tissues,  the  student  should  make 
careful  observations.  Attention  may  be  directed  especially  to  the  following  points : 
The  epidermis  at  the  sides  of  the  section  is  two-layered  and  consists  of  an  inner 
layer  of  cuboidal  cells,  the  anlage  of  the  Malpighian  layer  of  the  adult,  and  of 
an  outer  layer  of  very  thin  cells,  the  epitrichium,  the  nuclei  of  which  are  flattened 
and  appear  darkly  stained.  Toward  th^  medLli  line,  above  the  hind-brain  and 
T5eTow  T^c  *  Tore-brain,  the  epidermis  becomes  gradually  one-layered  and  much  thin- 
ner. The  mesenchyma  exhibits  three  principal  forms  of  cells:  First,  those  which  are 
equally  branched  in  all  directions,  and  represent  a  primitive  form  of  the  tissue. 
Such  may  be  found  in  the  neighborhood  of  the  basilar  artery.  Second,  the  elon- 
gated cells  of  the  cutis  zone;  and,  third,  the  cells  of  the  arachnoid  zone  above 
described.  The  blood-vessels  have  very  distinct  endothelial  walls  which  are  very 
thin,  being  thickened  only  to  furnish  space  lor  the  nuclei,  which,  unlike  those  of 
the  adult,  project  not  only  into  the  lumen  of  the  vessel,  but  also  against  the 
surrounding  mesenchyma.  The  red  blood-corpuscles  are  rounded  cells,  some- 
's oval,  not  infrequently  somewhat  distorted.  Their  nuclei  are  nearly  spherical 
a  number  of  fine  granules.  Mitotic  figures  are  quite  frequent.  A  few 
i  are  beginning  to  char  -.-  '  v  becoming  smaller  and  taking  the  stain 
-onipare  page  94).  °rvous  system  the  differentiation  of  the 

Vain   is   mo1-  n   in   the   fore-braip     but  even   in   the 

>  Tve-cells   and    the   young   neuroglia 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM. 


267 


cells  (neuroblasts  and  spongioblasts)  is  not  very  clear.  The  nuclei  are  only  just 
beginning  to  acquire  distinct  nucleoli,  such  as  would  be  characteristic  of  later  stages. 
The  nuclei  of.  the  tissues  differ  markedly  from  those  of  the  earliest  embryonic  stages, 
but  can  scarcely  be  said  to  have  assumed  in  any  of  the  tissues  adult  characteristics 


N.I  i. 


FIG.  rqr. — Pic     tj  SERIES  5,  SECTION  IQS. 

Card',  Card",  Cardinal  vein.     EC.  Ectoderm,    l-'.b.   '     te-brain.    /  ••/,    ;    ''undibular  gland.   A'. 5,  Trigeminal  gan- 
glion.    iV-7,8,  Acustico-fachl  ganglim.     N.<      Ganglion-  n  of  the  glosso-pharyngeal  nerve.    .AT.ic  " 
jug,  Jugular  ganglion  of  the  vagus  i  arm.     A'.ir,  Roo;                       •     ><•' -ory  nerve.     Md,  MedullaWf 
gata.     Ot,  Otocyst.    Sir.                                  'm.iii,    .       •!  vi  itriclp.     Ven.iv,  Fourth  ventricle.      X  - 


Section    through 
section   19  >,   and,   tii, 
bring,  out 'three    >oinis   n 


of  the    Otocyst    (Fig. 
ions  belo; 

the 


268  STUDY  OF  PIG  EMBRYOS. 

spinal  accessory  nerve,  N.n,  which  arises  from  the  cervical  (in  the  figure  upper) 
end  of  the  hind-brain  and  runs  forward  to  join  the  vagus  ganglion,  N.io  jug, 
the  jugular  ganglion  of  the  adult.  2°,  the  characteristic  relations  of  the  anterior 
cardinal  vein  to  the  trigeminal  ganglion,  AT.5.  The  vein  is  cut  twice,  Card'  and 
Ctf^sC,  for  it  curves  around  the  ganglion,  passing  on  the  inside  of  the  ganglion 
between  it  and  the  wall  of  the  brain.  The  original  vein  persists  throughout  life 
in-  this  position,  and  enlarges  into  the  cavernous  sinus  of  the  adult.  Inside  or 
mesially  of  the  seventh  to  twelfth  nerves  the  cardinal  vein  is  obliterated,  and 
is  replaced  by  a  new  vessel  produced  by  "island  formation"  outside  these  nerves, 
and  designated  as  the  vena  capitis  lateralis.  It  is,  as  it  were,  interpolated  in  the 
course'  of  the  original  vein,  and  this  interpolation  is  the  principal  factor  in  trans- 
forming the  embryonic  anterior-  cardinal  into  the  adult  jugular  vein.  In  the  12 
mm.  pig  the  vena  capitis  ^ateralis  is  formed  outside  the  otocyst  and  of  the  seventh 
and  eighth  nerves.  Later  it  extends  by  more  island  formations  outside  the  ninth 
to  twelfth  nerves  also.  The  jugular,  therefore,  is  to  be  defined  as .  the  anterior 
cardinal  vein  which,  by  successive  island  formations,  has  migrated  to  a  new  posi- 
tion outside  of  the  otocyst  and  cephalic  ganglia.  3°,  to  show  the  infundibular  gland, 
Inf,  a  small  evagination  from  the  ventral  floor  of  the  fore-brain,  F.b.  The  evagi- 
nation  is  really  hollow,  but  the  cavity  does  not  appear  in  the  section  figured.  It 
enters  into  very  close  relations  with  another  hollow  evagination,  which  springs  from 
the  dorsal  roof  of  the  oral  cavity  and  is  known  as  the  hypophysis.  The  infundib- 
ular gland  and  the.  hypophysis  become  intimately  associated  with  one  another  in 
their  further  development  and  give  rise  to  the  pituitary  body  of  the  adult,  the 
gland  becoming  the  posterior  lobe  the  hypophysis  the  anterior  lobe  of  that  or- 
gan. The-  hypophysis  may  be  best  studied  in  sagittal  sections  (see  page  292)  .<• 
.sent  section,  figure  191,  being  at  a  lower  level  than  figure  189,  passes  through' 
ventral  portion  of  the  hind-brain  and  shows  only  a  narrow  part  of  the.  cavity 
of  the  fourth  ventricle,  Ven.iv.  The  three  lavers  in  the  wall,  Md,  of  the  hind-brain 
are  very  distinct.  At  the  anterior  end  or  <-ne  hn.A  1  --^.v,  ^T-,P~—  ~  ^OK,  <•  light 
lines,  Str,  which  are  nv^eH  KV  n'-'-e-nbers.  These  lines  have  been  identified  as 
the  stria  a/>. .,.,.(.#'.  They  need  to  be  more  accurately  studied,  however,  for  they 
seem  r^Oier  to  be  fibers  of  the  lateral  root  of  the  facial  nerve.  Close  to  the  ante- 
-sio/'section  of  the  cardinal  vein,  Card",  appear  the  minute  fourth  and  third  nerves, 
which,  however,  are  not  indicated  in  the  figure.  Both  lie  close  to  the  wall  of  the 
vein  on  the  side  away  from  the  trigeminal  ganglioi  .  The  fourth  nerve  lies  nearer 
'  •  outside  of  the  embryo,  the  third  nerve  nearer  the  median  plane.  At  about 
^e  level  as  this  part  of  the  jugular  vein,  and  very  close  to  the  wall  of  the 
:*  situal  i  '  ->  loop  of  the  internal  Chrotic.  Lower  down,  but  not  ciose 
rn,  Js  the  section  of  the  lateral  jugular. 

""  ''•',  //    ntiil    Optic    i---j'igin"i:  "\    (Fig.     192). — The 
<>r  cervical   JCgi0n   of   the  spinal   cord,   on 
""    "-'rves.     In   this   and   the   three 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM. 


269 


sections  next  following  the  complicated  pharynx  appears  in  various  forms.  The 
general  shape  of  the  pharynx  has  been  described  with  the  aid  of  a  figure  of  a  wax 
model  of  the  pharynx  made  from  the  same  series  of  sections  from  which  these 


D.Z 


V.Z 


L.R.II. 


N.I  2. 


L.V. 


II 


EC. 


_     FIG.  192. — PIG,  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  249. 

Card,  Anterior  cardinal  vein.  Car.in,  Internal  carotid  artery.  cl.I,  First  or  auditory  gill-cleft.  D.Z,  Dorsal  zone 
of  spinal  cord.  EC,  Ectoderm.  H,  Anlage  of  cerebral  hemisphere.  L,  Lens.  L~K.II,  Lateral  rooi  of  th«- 
•  iith  nerve.  L.V,  Lateral  ventricle.  Mx.in,  inferior  maxillary  nerve.  NJ\  Facial  nerve.  X.y.petr* 
1'etrosal  ganglion  of  the  ninth  nerve.  A7".io,iT,  United  vagus  and  spinal  acdpsory  nerve.  A'.  12,  Hypo-i 
glossal  nerve.  Op,  Stalk  of  the  optic  ^vaginal ion.  Op.v,  Ophthalmic  vein.fPh,  Pharynx.  Ret.  Rttina. 
,  .^,  Ventral  zone  of  spinal  cord.  X  22  chams. 

figures   are   taken    (compare  Fig.    173,  p.    237).     The  shaperbi   the  pharynx  and   of 
its  four  pairs  of  latei      ^     ..ches  at  this  stage  is  remarkably' constant,  so  tha*  the  s* 
dent  is  not  likely  to  encounter  any  serious  difficulty  in  /dentif)] 
spinal    co-,-tl   is   oval   in   the   section.     Its   cavity   has   expinded   in    thi 


270  STUDY  OF  PIG  EMBRYOS. 

lateral   walls  are   quite   thick,   the  median   ventral   wall   is   thinner,   and   the   median 
dorsal  wall   (deck-plate)   is  very  thin.     The  three  primitive  layers  of  the  medullary 
tube  are  very  clearly  marked  out,  the  ectoglia  appearing  light,  the  ependymal  layer 
appearing   dark.     The   differentiation   is   much   more   advanced   on   the   ventral   side 
of  the  spinal  cord  than  on  the  dorsal  side,  and,  indeed,  it  is  only  in  the  ventral 
part  that  the  three  layers  are  perfectly  differentiated.     In  the  median  ventral  line 
we  have  the  floor-plate,  in  which  we  can  distinguish  only  two  zones,  while  in  the 
deck-plate  there  is  no  differentiation  of  layers  whatever.     The  spinal  cord  is  clearly 
divided  into  a  dorsal  zone,  D.Z,  and  a  ventral  zone,    V.Z,  on  each  side.     The  two 
dorsal    zones    are    connected  across  the  median  line  by  the  thin  deck-plate,  and  the 
ventral  zones  similarly  by  the  thin  floor-plate.     The  lower  or  ventral  limit  of  the 
dorsal   zone   is   marked   by   the   entrance   of   the   dorsal   or   ganglionic   root   and   by 
the  fibers,  which  represent  the  outgoing  lateral  roots.     In  the  actual  section  figured, 
the  lateral  roots,  L.R.n,  are  those  which  enter  into  the  formation  of  the  eleventh 
icrve.     The  true  dorsal  root  does  not  appear  in  the  figure.     Internally  the  division 
Between  the  two  zones  is  marked  by  the  lateral  angle  of  the  central  cavity  shown 
n  the  section.     In  the  dorsal  zone  the  differentiation  of  the  three  layers  has  made 
slight    progress.     In    the   ventral    zone,  however,    the    development    is    far    more    ad- 
vanced.    The  most    characteristic    feature    of    this    movement    is    the    growth    of    the 
inerea    or  neurone  layer,   which  increases  in  a  twofold  manner:  first,  by  encroach- 
ng   upon    the    inner   or    ependymal    layer;    and,  second,  by  the    growth  of   its  con- 
tituent   elements.     Examination   with   a   high   power   shows   at   once   that   the   cells 
tave   grown   very   much.     Their   nuclei   are   larger,    granular   in    appearance,    rarely 
/ith  any  indication  of   a  distinct  nucleolus.      Most  of   the  cells  are  neuroblasts  and 
ave  well-marked   protoplasmic   bodies,  finely  granular  in   texture.     They  have  -many 
f   them   already   produced  long,   slender   outgrowths   which   we   can   identify   as   the 
buraxons.     T"    order   to   study   the   distribution   of   the   neuraxons   and   the   form   of 
ae  ne1  Tibia         ii    ;.,  necessary  to  apply  the   Golgi  rapid  method,  by  which  it  can 
iiat  a  portion  of  the  neuraxons  is  distributed  entirely  within   the 
liar  iiile  another  portion  passes  out  to  form  ventral  roots,  one  of  which, 

[".12,    forming   part    of    the    hypoglossal    nerve,    is    shown    in    the    figure.     A    third 
m    of    the   neuraxons,    at   least    in    the    upper   cervical    region,    as    also    in    the 
edulla  oblongata,  passes'  out  to  form  the  lateral  roots.     The  positions  of  the  exits 
these  two  bundles   of  nerve-fibers   are   constant  and   characteristic.     The   ventral 
ot   always   passes-  out   from   the   middle   of   the   ventral   zone   about   half-way   be- 
een  the   median   floor-plate   and   the   dorsal   limit   of   the  zone.     The   lateral   root 
vays  passes  out  at  the  u]_^er  dorsal  limit  of  the  ventral   zone   and  immediately 
ow  the  point  of  enhance  trf"  the  true  dorsal  root.     Formerly  the  lateral  roots  were 
distinguished    from\the    dorsal    roots.    Following    downward    in    the    figure    we 
V  section  of  the  cardinal  vein,  Card,  just  inside  of  which  lies  the  common 
T,  of  the  un^ed  tenth  and  eleventh,  or  vagus  and  accessorius  nerves, 
the  lower,  part  of  the  petrosal   ganglion,   N.g.petr,   of  the  glosso- 


TRANSVERSE  SECTIONS  OF  EMBRl  L  12  MM.  271 

- 

pharyngeal  nerve.  Lower  down  and  nearer  the  ectoderm  lies  the  facial  nerve, 
N.'j,  situated  in  what  is  called  the  hyoid  arch  or  mass  of  tissue  intervening  be- 
tween the  first  and  second  gill-clefts.  The  hyoid  arch  is  further  marked  by  a 
bulge  in  the  external^ outline  of  the  section,  which  leads  down  into  a  deep  groove 
beyond  which  the  outline  of  the  section  again  rises  and  arches  forward  to  the  eye. 
This  groove  is  the  external  depression  of  the  first  gill-cleft  and  ultimatejy  is  trans- 
formed into  the  external  auditory  ifteatus.  The  position  of  this  groove  is  well  shown 
in  figure  166,  Au,  on  page  223.  Just  inside  the  auditory  groove  appears  the  outer 
end  of  the  first  or  auditory  internal  gill-pouch,  cl.I.  It  is  a  long,  oblique  slit, 
quite  narrow,  and  is  lined  by  a  layer  of  entoderm.  If  we  follow  it  along  through 
several  sections,  we  shall  find  that  higher  up  its  outer  end  comes  in  contact  with 
the  ectoderm  at  the  bottom  of  the  auditory  groove,  and  there  the  two  germ-layers, 
entoderm -and  ectoderm,  unite  to  form  a  single  membrane,  the  closing  plate  of  the 
gill-pouch.  Following  through  the  section  downward  in  the  series,  we  can  trace 
the  cleft  to  its  connection  with  the  pharynx,  Ph.  On  the  posterior  side  of  the 
cleft  we  find  the  internal  carotid,  Car.in.  Only  the  roof  of  the  pharynx,  Ph,  is 
cut,  so  that  it  occupies  but  a  small  area  in  the  section.  On  its  anterior  side  it 
shows  a  small  knob-like  projection  toward  the  floor  of  the  fore-brain.  This  is 
a  part  of  the  stalk  of  the  hypophysis:  Below  the  first  gill-cleft  appears  the  very 
large  and  conspicuous  inferior  maxillary  nerve,  Mx.in,  and  beneath  that  the  section 
of  the  small  ophthalmic  vein,  Op.v.  The  fore-brain  is  quite  complicated  in  shape, 
having  two  lateral  expansions,  L.  V,  of  its  cavity  which  are  destined  to  form  the 
lateral  ventricles.  The  walls,  H,  of  the  lateral  ventricles  are  the  anlages  of  the 
cerebral  hemispheres.  From  the  ventral  (in  the  figu  '  of  the  fore-brain 

spring   on   either  side   the   optic  stalks,   Op.     These    arc   hollow  lions   of   the 

brain,  which  expand  at  their  distal  ends  to  form  the  retina  of 
pigment  layer.     The  expanded  distal  ends  constitute  each  a  sort  of  cup,  of^'JliVl 
optic  stalk  is  the  stem.     The  cup  is  two-layered,  the  space  between  the  two  layers  be- 
ing a  prolongation  of  the  central  cavity  of  the  brain.     The  inner  of1  the  two  layers 
forms  the  retina  proper  and  is  considerably  thickened.     The  outer  layer  is  quite  thin 
and  is  already  quite  abundantly  laden  with  pigment  granules.     At  .the   edge  of   tb', 
cup  the  pigment   layer  passes  over   uninterruptedly  into  the   thick  retina  layer.     Jn 
the  cavity  of  the  optic  cup  lies  the  vesicular  lens,  L,  which  arose  from  an  evagma- 
tion  of  the  overlying  ectoderm.     The  vesicle  is,   however,  now  completely  separated 
from  the  layer  which  produces  it.     It  has  at  this  stage  a  very  largei  cavity,  and  in 
cent  sections  it  can  be  readily  seen  that  the  innc:   side  or  that  toward  the  brain 
-eady  thickening  and  changing  its  character  so  >;rn  the  main  part  of 

adult  lens.     The  thickening  depends  chiefly  upon  the  rapid  and  enyimon 
of  the   epithelial  cells  of  this  part  of  the  vesicle,  so   that  they 

the  so-called  fibers  of  the  adult  K-ns,  (.        ndull 

Section   through  -«id  Gill-Cleft  <.     /  Oral  .  c\cl   oi 

ihis  section  is  such  that   tn     head  is  cut  separately  jnd  appears  in  sermon  without 


STUDY  OF  PIG  EMBRYOS. 


EC. 


L.V. 


N.cerv.i. 


Ao.D. 


Olf. 


P.M. 


FIG.  193.     PIG,   12.0  MM.     TRANSVERSE  SERIES  5,  SEC-HOX  292. 

',  Descending  aorta.  Card,  Anterior  cardinal  vein,  car.in,  Intern.-)!  Carotid  artery.  cl.II,  Second  gill-clef' 
EC.  Ectoderm.  H,  Anlage  of  cerebral  hemispheres.  L.gr,  Lachrymal  groove.  L.V,  Lateral  ventricle  of 
brain.  W  •?.',,  \I;mdibular  arch.  MX,  Maxillary  process.  My,  Myotome.  nch,  Notochord.  N.cerv.i, 
i-  irst  'cervical  nerve.  N.j,  Facial  nerve.  A". 9,  Glosso  pharyngeal  nerve.  Ar.icv  Vagus  nerve.  Ar.n, 
Spinal  accessory  nerve.  O.F,.()ral  fissure  or  s[)ace  MRwi-en  .he  head  and  mandibles.  Olf,  Olfactory  pit. 
Pk,  Pharynx.  P.M,  Pia  mater.  Sp.c,  Spinal  cord.  t<  22  di.-, 


f 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  273 

connection  with  the  body  of  the  embryo.  The  space  O.F,  between  the  head  piece 
and  body  piece,  may  be  designated  as%  the  oral  fissure,  since  it  is  into  this  space 
that  the  mouth  opens.  In  general  there  is  considerable  resemblance  between  this  and 
the  section  last  described,  but  in  the  present  section  the  eyes  have  disappeared 
and  we  get  the  first  indications  of  the  nasal  pits,  Olf.  That  on  the  left  side  of 
the  body  shows  a  trace  of  the  cavity  of  the  pit.  The  posterior  part  of  the  pharynx, 
Ph,  is  cut  in  the  section,  instead t  of  the  anterior  part  as  in  figure  192.  The  first 
gill-cleft  does  not  show,  but  the  second  cleft,  cl.II,  does.  It  lies  posterior  to  the 
first  cleft  and  therefore  appears  higher  up  in  the  figure.  The  spinal  cord,  Sp.c, 
shows  the  same  general  structure  as  in  the  previous  section.  On  either  side  of  it 
may  be  seen  the  small  and  inconspicuous  root  of  the  eleventh  or  accessory  nerve. 
It  could  not  be  properly  represented  in  the  figure.  Some  distance  below  the 
cord  lies  the  small  circular  section  of  the  notochord,  which  differs  so  slightly  .in 
staining  from  the  surrounding  mesenchyma  that  it  cannot  be  well  made  out 
without  the  use  of  a  higher  magnifying  power.  It  is  enclosed  by  a  distinct  mem- 
brane which  is  thick  enough  to  produce  a  double  outline,  and  contains  a  consid- 
erable number  of  scattered  nuclei,  which  are,  however,  nowhere  much  crowded. 
The  nuclei  are  round  in  form,  decidedly  larger  than  those  of  the  surrounding 
mesenchyma,  granular,  and  containing  each  several  more  conspicuous,  darkly 
staining  granules.  There  is  a  very  slight  gathering  of  mesenchymal  cells  about 
the  notochord,  as  if  to  form  the  anlage  of  a  sheath.  Just  below  the  noio< 
there  is  a  broad  band  of  somewhat  daTker  staining,  due  to  a  greater  conden? 
of  the  mesenchyma  in  that  region,  and  this  condensation  represents  the  beginning 
of  the  formation  of  the  vertebral  structures.  On  either  side  we  find  the  trans- 
formed myotome,  My,  or  anlage  of  the  striated  muscular  tissue.  This  tissu 
produced  from  the  'secondary  somites  of  earlier  stages.  The  cells  have  now  sepa- 
rated from  one  another,  .  have  lost  their  distinctly  segmentai  grouping,  and 
begun  to  elongate  into  true  muscle-fibers.  All  that  can  be  distinguished 
the  low  power  is  the  somewhat  darker  appearance  of  this  part  of  the  section, 
to  the  great  crow'' at:  of  the  nuclei.  Between  the  muscular  anlage  and  the  noto- 
chord '  shows  a  portion  of  the  first  cervical  nerve,  N.cerv.i,  and  just 

;    nerve   is   a   small   blood-vessel   not   represented   in   the   figure.     Th 
a    similar  blood-vessel    symmetrically    placed    on    the    opposite    side.     They    ar^    the 
small  vertebral  arteries.     The  anterior  cardinal   veins,  Card,   are  large  and  conkpieu- 
ous   vessels,   but   despite   their  size   they   have  merely   endothelial   walls   and   thl; 
no    condensation    of    thb^ mesenchymal    cells    around    them,    although    such    a  I 
densation  is  to  take  place  later  to  form  the  anlages  of  the  muscular  and  char 
ive-tissue   coats    (media   and  adventitia)    of    the   adult.     On   the   dorsal   side   o*     :'e 
cardinal  vein   and   close   to  it  is   a   light   spot   in  which   can   be   easily  distingu  ' 
\vith   the  high  power,  several  more  or  i-=s  distinct  bundles  of  nerve-fiber?  whk 
separated    from    one   another    by    mesenchyma  1    cells.     For    this    reason    it    is 
\vhat  difficul1    to  recognize  this  ner\       viih  ti  T  or  to  at  it 

1 8 


274-  STUDY  OF  PIG  EMBRYOS.      . 

figure.  On  the  ventral  side  of  the  vein  there  appears  a  darkly  stained  mass, 
N.io,  the  nodosal  ganglion  of  the  vagus  nerve,  and  outside  of  this  ganglion  is 
the  section  of  the  spinal  accessory  nerve.  Immediately  below  the  nodosal  ganglion 
we  have  the  internal  carotid  artery,  car.in.  A  little  to  the  inside  of  the  jugular 
is  a  small  vessel,  Ao.D,  of  great  morphological  importance.  The  corresponding 
vessel  appears  on  the  opposite  side.  Although,  very  small,  this  vessel  has  a  dis- 
tinct coat  of  condensed  mesenchyma  around  its  endothelium.  The  two  vessels 
are  the  descending  aorta,  which  have  almost  completely  aborted,  and  in  slightly 
older  specimens  will  be  found  to  have  disappeared  altogether.  The  descending 
aortae  are  the  longitudinal  trunks  by  which  the  dorsal  ends  of  the  five  aortic 
arches  of  early  stages  are  connected  together.  The  portion  shown  in  this  section 
is  the  part  of  the  descending  aorta  between  the  tops  of  the  third  and  fourth  aortic 
arches.  The  relations  are  shown  in  the  reconstruction  (Fig.  172).  The  pharynx, 
Ph,  is  narrow  in  its  dorsal  ventral  diameter,  but  wide  transversely,  and  offers  the 
very  characteristic  yoke-shaped  figure  in  the  section.  The  distal  portions  of  the 
second  gill-clefts  are  shown,  and  they  .appear  disconnected  from  the  pharynx, 
the  connection  occurring  in  sections  higher  up.  Each  cleft  is  somewhat  slit-like, 
so  that  its  cavity  is  an  oblique  fissure  and  somewhat  parallel  in  position  to  the 
first  cleft  (Fig.  192).  Both  the  pharynx  and  the  gill-clefts  are,  of  course,  lined 
throughout  by  entoderm,  which  forms  a  sharply  defined  layer  crowded  everywhere 
with  nuclei,  which  are  of  about  the  same  size  as  those  of  the  surrounding  mesen- 
chyma. In  the  pharynx  the  entoderm  is  somewhat  thinner  on  the  dorsal  than 
on  the  ventral  side.  In  the  clefts  it  is  thicker  than  in  the  pharynx  proper,  and 
especially  in  the  clefts  it  may  be  observed  that  the  mitotic  figures  always  occupy 
a  superficial  position.  On  the  dorsal  side  of  the  cleft  is  a  very  small  blood-vessel, 
near  which,  with  a  higher  power,  one  may  see  a  small  nerve,  and  nearby,  but 
more  dor-salward,  a  second  nerve.  Both  of  these  are  branches  of  the  glosso- 
pharyngeus,  and  lie  behind  the  cleft.  They  are,  therefore,  termed  the  post-trematic 
branches.  Below  the  cleft  and  somewhat  on  its  median  side  is  a  similar  third 
nerve-branch,  the  pre-trematic  of  the  glosso-pharyngeus,  running  in  front  of  the 
cleft.  The  outline  of  the  embryo  forms  a  rounded  eminence  outsidf^of  the  second 
cleft;  it  represents  in  part  the  hyoid  arch.  In  the  midst  of  .the  mesodenii  ".$  this 
appears  a  •  light  area  with  a  few  nerve-fibers,  the  end  of  the  facial  nerve,  A . , 
The  mandibular  arch  or  process,  Mdb,  is  very  distinct  and  prominent.  It  is 
separated  from  the  hyoid  arch  by  a  deep  external  notch,  which  corresponds  to 
the  external  first  or  auditory  cleft.  In  the  interior  of  trie  mandibular  process 
therct  are  light  spaces  differing  in  their  exact  distribution  on  the  two  sides  of 'the 
These  spaces  contain  n  live-fibers  and  they  represent  the  inferior  maxil- 
.  We  now  come  to  the  oral  fissure,/0.jF,  which  separates  the  body  * 


he  head.     In  the  head  portion  of  the  section  we  have  the  maxillary  process* 
•hich  is  separated   in  part  from  the  rest  of  the  head  by  the  deep  lachrymal 
L.gr.     On  either  side    there/shows    a   shaving    from    the    epithelium    of    tse  ' 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  27 5 

olfactory  chamber,  Olf.  The  fore-brain  has  expanded  laterally,  L.V,  to  form  the 
lateral  ventricles,  the  walls  of  which,  H,  are  the  anlages  of  the  cerebral  hemispheres. 
On  the  dorsal  side,  which  is  the  lower  side  in  the  figure,  the  hemispheres  project 
somewhat,  leaving  a  median  space  between  them.  This  median  space  is  filled 
with  mesenchyma,  which  may  already  be  regarded  as  the  anlage  of  the  falx.  In 
the  tissue  of  the  falx  are  two  very  small  blood-vessels,  the  forward  prolongations 
of  the  lateral  jugulars,  which  are  to  unite  to  form  the  median  superior  longitudinal 
sinus.  In  the  previous  section  these  vessels  also  reappear,  but  are  already  united 
(Fig.  192).  In  the  median  dorsal  line  the  wall  of  the  fore-brain  is  thin  and  shows 
a  characteristic  notch.  Close  to  the  surface  of  the  fore-brain  there  is  a  very  dis- 
tinctly marked  vascular  layer,  the  commencing  pia  mater,  P.M,  and  with  a  high 
power  it  can  be  easily  seen  that  the  differentiation  of  the  arachnoid  zone  has 
already  begun. 

Section    through  the   Third  Gill-Cleft  and  Nasal  Pits  (Fig.    194). — In  this  section 
the  head  is  clearly  separated  by  a  considerable  space  from  the  rest  of  the  section. 
The   transverse   diameter   of   the   embryo   is   here   much   less   than   higher   or   lower, 
so    that    the    section    as    ay-whole    seems    somewhat    narrow.     It    shows    the    entire 
length    of    the    third    gffl^cleft,   cl.iii,   exhibiting,  on   one    hand,   its   connection  with 
the  median  pharynx,  and,  on  the  other  hand,  its  'dorsal  extremity,  where  its  ento- 
derm  joins  the   ectoderm.     The   external   outline  of  the  embryo  makes  a  deep   de- 
pression  outside   the   end   of  the   third  cleft.     This   depression   is   the   cervical   sinus 
(compare   Fig.    166,   C.S;  pig   of    10   mm.). .   In   the   section   the   cervical   sinus   dis- 
plays   a    narrow    downward    prolongation.     If    followed    through    in    the    series    of 
sections,   this  prolongation,   which  is  on   the  inside  of  the   hyoid  arch,   Hy,   will  be 
found    to    connect    with    the    second    cleft.     The    spinal    cord,   Sp.c,    presents    essen- 
tially  the   same   structure   as   in  116   and    117.     Our  section   passes   through 
the  roots   of   the   second   cervical  'frv.2,   and   shows  both   the  dorsal   gan- 
glion   and    the    ventral   rooi  I   zone.     These   two   roots   join 
and    form    the   nerve-trunk,  'ivides,    sending   one 
branch   vertically   upward   into   a   m;:                                   <         cells    (the   aniage   of   the 
dorsal     musculature)     and     a     ventral     branch     which     descfcods     a!rpo> 
toward  the  pharynx.     Just  inside  of  this  ventral  branch              ''^^H 
vertebral  artery,  Art.v.     Between  the  dorsal  summit  of  the  ganglio- 
cord    there    is    a   minute   bundle    of   nerve-fibers   not   shown   in    the   fi^. 
fibers    constitute  the  commissural  trunk  of    the  eleventh  nerve.     The  third  gill- 
cl.iii,   is  cut  almost  symmetrically,   and   extends   from  t-he  median   line  to   the   edge 
of  the   section.     It  is  lined  throughout  by  the  entoderm,  which  at  the  end  of  the 
cleft    on    each   side    has   met   and    fused   with    the   ectoderm   to   form   the   epithelial 
membrane,    the   closing   plate.     The   membrane   apparently   normally   remains    intact 
in    mammals.     In    the    ichti:                              -mbrane    bee  1        !     ;  --'rxg    em- 

•  ft  is   OP  -he  cleft 

'he  id'.-rgonc    a    special  .on 


276 


STUDY  OF  PIG  EMBRYOS. 


the  side  of  the  cleft  toward  the  head.  This  structure  is  the  anlage  of  the 
nodulus  thymicus  and  is  already  penetrated  by  small  blood-vessels  which  are 
perhaps  not  capillaries,  but  sinusoids.  The  fate  of  nodulus  is  uncertain;  it 
probably  forms  the  head  of  the  thymus,  and  not  the  carotid  gland  as  some 


Fourth  aorti 


FIG.  194. — PIG,  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  321. 

Fourth  aortic  arch.  Art.v,  Vertebral  artery.  AH,  External  auditory  cleft.  Card,  Anterior  cardinal  vein. 
cl.iii,  Third  internal  gill-cleft.  G.nod,  Ganglion  nodosum.  Hy,  Hyoid  arch.  L.V,  Lateral  ventricle. 
.\'<i.  Nasal  pit.  N.cerv.i,  First  cervical  nerve.  N.ceru.2,  Second  cervical  nerve.  N.II,  Spinal  accessory 
nerve.  IV.  12,  Hypoglossal  nerve.  N.od,  Nodulus  thymicus.  Nv,2,  Main  trunk  of  second  cervical  nerve. 
Ol.pl,  Olfactory  plate.  R.ex.n,  Ramus  exter,nus  of  the  spinal  accessory.  Sp.c,  Spinal  cord.  Th\r, 
Thyroid.  Tr,  Trachea.  X  22  diams. 

have    suggested.     Thfc   student    should    clearly    understand    that  the  median  region 

of      the       third      gill-cleft      is     really    the     pharynx     proper.      From     its     median 

ventral     line  '  arises     the    beginning    of     the    trachea,    Tr,    which    shouU,    perhaps, 

designated   as   the   anlage   of   the   larynx.     The   entoderm   yxtends   down 


! 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  277 

in  the  median  line  for  a  considerable  distance,  making  a  figure  which,  in  the 
section,  is  shaped  somewhat  like  an  inverted  spear-head.  In  the  center  of  the 
section  appears  a  small  cavity.  Farther  down  toward  the  lungs  we  have  only  an 
epithelial  plate  with  no  cavity  observable  in  it  (Fig.  195,  Tra),  the  entoderm  of 
the  trachea  at  this  stage  forming  a  solid  cord.  Ventrad  from  the  trachea,  in 
the  median  region  and  between  the  two  aortic  arches,  is  a  small,  irregular,  deeply 
stained  mass  of  cells,  Thyr,  the  anlage  of  the  thyroid  gland.  These  cells  are  ento- 
dermal,  the  anlage  having  been  developed  by  a  downgrowth  of  the  epithelium  of 
the  floor  of  the  pharynx,  although  at  the  present  stage  the  original  connection  with 
the  pharynx  has  been  lost.  The  anlage  is  now  isolated  from  its  parent  germ-layer 
and  is  imbedded  in  mesenchyma.  It  is  solid,  for  the  cavities  of  the  thyroid  follicles 
are  not  developed  until  considerably  later.  Just  above  the  third  gill-cleft  may 
be  seen  a  large,  darkly  stained  mass,  G.nod,  the  ganglion  nodosum  of  the  vagus 
nerve.  Immediately  above  it  is  a  section  of  the  anterior  cardinal  vein,  Card. 
Between  the  ganglion  and  the  vein  is  a  bundle  of  nerve-fibers  representing  the 
twelfth  or  hypoglossal  nerve,  which  reappears  again  in  the  section  below  the 
pharynx,  at  N.I2.  The  reason  for  this  double  appearance  of  the  hypoglossal 
nerve  may  be  seen  readily  by  examination  of  the  reconstruction  (Fig.  178).  Close 
to  the  ganglion  on  its  outer  side  is  the  section  of  the  spinal  accessory  nerve,  N.n. 
A  little  above  the  jugular  vein  is  the  section  of  the  first  cervical  nerve^  N.cerv.i, 
laterad  from  which  is  the  external  branch,  R.ex.n,  of  the  spinal  accessory  nerve. 
This  branch  in  the  adult  innervates  the  sternocleidomastoid  and  trapezius  muscles. 

The  lower  part  of  the  figure  represents  the  section  of  the  head  and  shows 
the  two  nasal  pits,  Na,  closed  toward  the  mouth  side  by  the  olfactory  plate,  Ol.pl, 
the  epithelial  membrane  somewhat  resembling  the  closing  plate  of  a  gill-cleft, 
but  it  is  formed  by  a  fusion  of  the  ectoderm  on  the  two  sides  of  the  opening 
of  the  nasal  pits.  When  the  nasal  pits  are  first  formed,  they  are  open  throughout 
their  whole  extent.  The  formation  of  the  •  y  plate  is  the  first  step  toward 

the    separation    of   the  two  nasal  cavities  from  i  ravity.     In  later  stages  this 

plate   disappears,   and  its   forward   portion   is   replace  m:hy  ni,   so   that   the 

separation   of   the   nasal   and   oral    cavities   is   permanent  posterior   portion 

of   the   membrane   becomes   first    very    thin,    and    finally   di^  r'-tiier,    *'.,.  - 

establishing   a   secondary    connection    between    the  nose   and    mo, 
chamber.     On   the   dorsal   side   of   the  nasal   pits    (below  in  Fig.    1(4.. 
hemispheres   are   cut   separately,   their   darkly   stained   walls   bounding   on    <_ 
the  large  lateral  ventricle,  L.V. 

Section  through  the  Fourth  Gill-Cleft  (Fig.  195).— Of  the  entodermal  gill-cleft? 
or  -pouches  the  fourth  is  by  far  the  smallest,  and  as  it  appears  in  sections  (rl.l  \' : 
is  inconspicuous.  The  section  figured  differs  by  two  striking  features  from  'those 
of  the  series  above  described:  first,  because  the  head  is  no  longer  included;  and, 
second,  because  the  cardiac  structures  are  beginning  to  show.  On  the  dorsal  side 
the  spinal  cord  "is  cut  at  the  ievei  of  U.L  ganglion,  (7. 3,  of  the  third  cervical  nerve. 


278 


STUDY  OF  PIG  EMBRYOS. 


The  dorsal  root  of  the  ganglion  joining  the  spinal  cord,  Sp.c,  is  shown  on  both 
sides  of  the  section,  and  the  nerve  itself  also  appears,  being  best  shown  on  the 
left  side  of  the  embryo,  where  a  short  piece,  R.D.-$,  of  the  ramus  dorsalis  is 
included  and  a  much  longer  piece,  ^.^.3,  of  the  ramus  ventralis.  Just  inside  of 
the  nerve  at  the  level  of  the  notochord,  Nch,  is  the  cross-section  of  the  vertebral 
artery.  On  the  right  side  of  the  embryo  the  section  passes  through  a  portion  o.f 


Sp.c. 


FIG.  195. — PIG,  I3--.0  MM.     TRANSVERSE  SERIES  5,  SECTION  353. 

Ao,  Aorta.  Ao.4,  Fourth  aortic  a>vh.  Au.d,  Right  auricle.  Au.s,  Left  auricle.  Card,  Anterior  cardinal  vein- 
Cos,  Coelom.  d.IV,  Four*"  gill-pouch.  £.3,  Ganglion  of  third  cervical  nerve,  msth,  Mesothelium.  N . 
ctrv.2,  Second  cervical  nerve.  Nch,  Notochord.  N.IO,II,  United  vagus  and  spinal  accessory  nerves.  P. A, 
Pulmonary  artery.  PA,  Pharynx.  R.D. 3,  Dorsal  ramus  of  the  third  cervical  nerve.  R-V.$,  Ventral  ramus 
••  '  cervical  nerve.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  Sym,  Sympathetic  nerve  chain.  Tra, 
Trachea.  Ve,  Vein  to  lower  jaw.  X  22  diams. 

the  second  cervical  nerve,  N.cerv.2.  The  anterior  cardinal,  Card,  is  a  very  large 
vessel.  Close  to  its  ventral  wall  appear  a  few  fibers  which  represent  the  first  cer- 
vical rterve,  but  they  are  too  indistinct  to  be  represented  in  the  figure.  They  may 
easily  be  found  with  the  higher  power.  In  the  median  plane  is  the  crescent-shaped 
section  -of  the  pharynx,  Ph.  Between  the  jugular  vein  and  the  pharynx  lies  the 
fourth  aortic  arch,  Ao.^.  The  right  and  left  arches  are  at  this  stage  about  equal  in 
size,  although  the  left  arch  is  destined  to  form  the  main  aortic  arch  of  the  body, 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  279 

and  only  a  portion  of  the  right  arch  will  persist  to  form  a  portion  of  the  stem  of 
the  pulmonary  artery.  The  figure  indicates  the  manner  in  which  these  aortic  arches 
pass  up  from  the  heart  below  on  either  side  of  the  pharynx.  A  little  above  the 
aortic  arch  on  either  side  may  be  seen  a  small,  round  spot,  Sym,  which  is 
somewhat  conspicuous  on  account  of  its  deeper  staining.  It  is  a  section  of  the 
cervical  sympathetic.  Examination  with  a  higher  power  shows  that  it  consists  of 
somewhat  crowded  cells,  some  of  which  have  larger  nuclei.  These  are  the  neuro- 
blasts.  The  mesenchymal  cells  immediately  around  the  anlage  are  disposed  about  it  in 
somewhat  concentric  lines.  Between  the  cardinal  vein  and  the  aortic  arch  is  situated 
the  large,  conspicuous  nerve-trunk,  N.  10,1*1,  constituted  by  the  united  vagus  and 
spinal  accessory  nerves.  Below  this  double  nerve  is  a  blood-vessel,  Ve,  a  branch 
of  the  cardinal  vein.  This  vessel  drains  the  tongue  and  facial  region  of  the  em- 
bryo. It  is  homologous  with  the  inferior  jugular  vein  of  lower  vertebrates,  and 
in  mammals  gives  rise  to  the  lingual  and  facial  veins  of  the  adult,  and  in  some 
species  forms  the  external  jugular,  but  the  human  external  jugular  is  a  secondary 
anastomosis  between  the  linguo-facial  and  the  junction  of  the  internal  jugular  and 
subclavian  veins.  The  homologies  between  this  vein  and  those  of  the  adult  have 
not  yet  been  worked  out.  Returning  now  to  the  pharynx,  Ph:  on  the  right  side 
the  prolongation  of  the  pharynx  to  join  the  fourth  cleft  can  be  clearly  followed;  on 
the  left  side  of  the  embryo,  the  right  of  the  figure,  the  fourth  cleft,  cl.IV,  does 
not  display  its  connection  with  the  pharynx,  but  is  a  separate,  small,  epithelial 
cavity  lined  by  a  cylinder  epithelium.  Underneath  the  pharynx  appears  a  vertical 
plate,  Tra,  formed  by  the  entoderm  of  the  trachea.  This  plate  is  thinnest  in  the 
middle,  somewhat  wider  toward  the  top  and  bottom  of  the  section.  It 
solid,  except  for  a  minute  cavity  at  its  dorsal  end.  This  minute  ity  may  be 
traced  from  the  opening  of  the  glottis  through  the  series  of  sections  down  until  it 
becomes  connected  with  the  comparatively  large  cavities  of  the  developing  bronchi 
of  the  lung.  Below  the  pharyngeal  region  descends  the  thick  somatopleure,  Som, 
which  encloses  the  pericardial  coelom,  Cce,  in  which  the  heart  is  lodged.  The  inner 
surface  of  the  somatopleure  is  covered  by  the  thin  mesothelium,  msth.  Of  the 
cardiac  structures  we  note  first  the  section  of  the  main  aorta,  Ao,  and  of  the 
pulmonary  aorta,  P. A,  and  finally  small  sections  of  the  uppermost  part  of  the  .two 
auricles,  Au.d  and  Au.s.  More  of  the  left  auricle  is  included  in  the  section 
of  the  right. 

Section  through  the  Anterior  Limbs  and  Heart  (Fig.  196). — The  section  figured  is 
much  lower  in  the  series  than  the  last  and  was  selected  in  order  to  illustrate  the 
anterior  limb-buds,  the  common  cardinals,  and  the  heart.  The  position  and  shape 
of  the  limb-buds  are  sufficiently  shown  in  figure  166.'  The  section  demonstrates  that 
the  limb-bud  is  formed  chiefly  by  a  dense  mass  of  mesoderm  covered  by  a  thin 
layer  of  ectoderm.  The  mesoderm  consists  of  very  much  crowded  fells  in  which  it 
is  very  difficult  to  recognize  any  distinct  differentiations,  yet  it  is  probable  that 
here  are  mingled  both  true  mesenrhymal  cells  and  cells  which  originally  belonged  to 


280  STUDY  OF  PIG  EMBRYOS. 

the  muscle-plates,  but  which  have  now  broken  apart  and  are  developing  singly  into 
muscle-fibers.  In  certain  amphibia  the  cells  from  the  muscle-plate  can  be  distin- 
guished from  the  mesenchymal  cells  of  the  limb,  and  what  we  know  of  the  devel- 
opment of  the  muscles  in  amniota  confirms  the  view  that  striated  muscles  and  mesen- 
chyma  are  genetically  entirely  distinct.  No  skeletal  elements  whatever  have  yet  arisen 
in  the  limb.  We  have  here  a  striking  illustration  of  the  fact  that  the  skeleton  is 
very  late  in  its  development,  and,  embryologically  speaking,  is  in  no  sense  the  frame- 
work upon  which  the  body  is  built  up,  but  rather  a  late  supplementary  develop- 
ment. The  main  morphological  features  in  all  parts  of  the  embryo  are  entirely 
fixed  by  the  soft  tissues  before  the  skeletal  structures  arise.  Both  nerves  and  blood- 
vessels have  grown  into  the  limb.  The  nerves  are  the  ventral  branches  of  the 
spinal  nerves.  Several  of  these  unite  together  and  form  the  brachial  plexus,  one 
part  of  which,  Br^Plx,  is  shown  in  the, section.  In  the  present  embryo  this  nerve- 
trunk  includes  fibers  derived  from  both  the  sixth  and  seventh  cervical  nerves.  Just 
above  the  nerve-trunk  is  the  section  of  the  subclavian  or  axillary  vein,  which  is  a 
branch  from  the  jugular.  The  dorsal  region  of  the  embryo  is  relatively  larger  at 
the  level  of  this  section  than  higher  up,  owing  chiefly  to  the  great  development  of 
the  mesoderm.  The  spinal  cord,  Sp.c,  resembles  that  in  figure  195,  but  is  both  larger 
and  more  differentiated.  On  the  left  side  of  the  embryo  the  fundamental  morpho- 
logical characteristics  of  the  spinal  nerve  are  well  illustrated  in  this  section.  The 
dorsal  root,  D.R,  bears  the  ganglion,  G,  which  joins  the  dorsal  zone  of  the  spinal 
cord.  The  fibers  of  this  root  are  produced  from  the  cells  of  the  ganglion  and  grow 
from  the  ganglion  into  the  spinal  cord.  Other  fibers  from  the  same  cells  grow  out 
K  the  opposite  direction  and  form  the  nerve-trunk  or  root  which  descends  from  the 
ganglion  in  a  nearly  vertical  direction.  The  ventral  root,  V.  R,  arises  from  the  ven- 
tral zone,  takes  an  oblique  course,  and  joins  the  dorsal  root  a  little  below  the  level 
of  the  spinal  cord  to  form  a  single  nerve-trunk,  which,  however,  soon  subdivides  into 
its  two  primary  branches.  The  first  or  dorsal  branch,  R.D,  bends  at  an  acute  angle 
upward  and  outward.  The  second  or  ventral  branch,  ramus  ventralis,  continues 
dowttWP^d  and  curves  into  the  limb.  Owing  to  this  curvature,  in  order  to  follow 
its  cayrse  the  nerve  must  be  traced  through  adjacent  sections.  If  this  is  done,  the 
venial  ramus  will  be  found  to  take  part  in  the  formation  of  the  brachial  plexus. 
Some-  distance  below  the  spinal  cord  is  the  small  notochord.  Farther  down,  but 
also  in  the  median  line,  appear  two  small  rings  of  epithelium.  Of  these,  the  smaller 
upper  one,  (E,  is  the  entodermal  lining  of  the  oesophagus,  and  the  larger  lower  one 
is  the  entodermal  lining  of  the  trachea.  Around  each  of  these  rings  there  has  already 
occurred  a\  slight  condensation  of  the  mesenchyma,  the  first  step  toward  the  ulti- 
mate differentiation  of  the  submucous  and  muscular  coats  of  the  oesophagus  and 
trachea.  The  \entoderm  of  both  the  oesophagus  and  trachea  is  a  moderately  thick 
layer  composed  of  elongated  cells,  the  nuclei  of  which  are  distributed  at  various 
levels,  but  so  as  to  leave  the  superficial  portion  of  the  layer  comparatively  free.  It 
in  to.is  superficial  portion  that  the  mitotic  figures  always  occur.  On  the  ven- 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM. 


281 


tral  side  of  the  trachea  and  somewhat  toward  the  left,  but  quite  close  to  it,  ap- 
pear two  small  blood-vessels,  the  pulmonary  arteries.  They  are  so  small  that  their 
lumina  scarcely  show  in  the  figure.  Two  sections  nearer  the  head,  the  two  ves- 
sels are  found  united  in  a  single  stem.  Opposite  the  arteries  below  the  trachea 


Sp.c. 


D.R. 


Nch 


F. 


Som. 


FIG.  196. — PIG,  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  470. 

Ao.S,  Left  descending  aorta.  Au.d,  Right  auricle.  Br.Plx,  Brachial  plexus.  C.C.S,  -Vena  cardinalis  communis 
sinislra.  D.R,  Dorsal  root  of  spinal  nerve.  F,  Cardiac  fissure.  G,  Spinal  gangfion.  L,  Anterior  limb-bud 
Nch,  Notochord.  Nv,  Branch  of  brachial  plexus.  (E,  (Esophagus.  R.D,  Ramus  dorsalis  of  spinal  nerve. 
S.a.c,  Septum  of  the  auricular  canal.  ScLV,  Subclavian  vein.  Som,  Somatopleure.  Sp.c,  Spinal  cord. 
S.s,  Septum  superius.  Tra,  Trachea.  Val,  Auriculo-ventricular  valve.  Ven.S,  Left  ventricle  of  the  heart 
V.R,  Ventral  root  of  spinal  nerve.  X  22  diams. 


a    minute    opening,   not    shown    in   the    figure,    marks    the   tip   of    th 
chamber.     To    the    right    and    left    of    the    oesophagus    appear    th 
of  the  two  descending  aorta-,  of  which  the  left,  Ao.S,  is  alread' 
the  right.     Ultimately  the  greater  part  of  the  right  •/ 


iwal 


282  STUDY  OF  PIG  EMBRYOS. 

of  the  adult  being  formed  from  the  left  aorta.  Lower  down  in  the  series  the  two 
descending  aortse  unite  to  form  the  single  median  dorsal  aorta.  The  common 
cardinals,  C.C.S,  are  two  enormous  venous  trunks  which  deliver  the  blood  to  the 
heart.  They  lie  symmetrically  placed  to  the  right  and  left  of  the  oesophagus  and 
trachea.  They  extend  from  the  level  of  the  descending  aortae  downward  and  inward 
to  the  level  of  the  heart.  The  vein  of  the  left  side,  C.C.S,  is  almost  symmetrical 
with  its  fellow  of  the  right  side,  though  it  has  no  direct  communication  with  the 
heart;  but  by  following  down  through  the  series  of  sections  the  student  can 
observe  that  the  left  common  cardinal  connects  across  with  the  corresponding  vein  of 
the  right  side.  The  right  vein  opens  directly  into  the  right  auricle,  Au.d,  of  the 
heart.  All  of  the  venous  blood  is  collected  at  this  stage  by  the  common  cardinals, 
except  that  which  comes  through  the  liver.  The  common  cardinals  are  formed  by 
the  union  of  the  jugular  or  anterior  cardinal  vein  from  the  head  with  the  posterior 
cardinal  vein  from  the  body.  The  opening  of  the  right  vein  into  the  auricle  of 
the  heart  is  guarded  by  two  small  flaps  or  valves.  The  lower  part  of  the  section 
is  occupied  by  the  large  heart  lying  in  the  pericardial  chamber.  The  body-wall, 
Som,  or  somatopleure,  which  forms  the  outer  covering  of  this  chamber,  is  quite  thin 
and  without  a  trace  of  muscular  or  skeletal  structures.  It  consists  of  three  distinct 
layers — the  external  ectoderm,  the  middle  mesenchyma,  and  the  internal  mesothelium. 
The  mesothelium  is  a  thin  layer  of  cells  which  persists  throughout  life  and  is 
known  in  the  adult  as  the  pericardial  epithelium.  In  the  present  section  it  is  easy 
to  follow  this  layer  from  the  somatopleure  past  the  common  cardinals  on  to  the 
heart  and  completely  around  the  outside  of  the  heajrt  itself.  Everywhere  it  forms 
the  covering  or  boundary  of  the  ccelom  of  the  pericardium.  In  later  stages  this 
mesothelium  will  have  an  especial  layer  of  connective  tissue  close  under  it.-  The 
layer  of  connective  tissue,  together  with  the  mesothelium,  constitutes  the  pericardial 
membrane  of  descriptive  anatomy.  The  essential  fundamental  relations  -  of  this 
membrane  may,  therefore,  be  easily  understood  from  the  present  section.  From 
the  study  of  the  adult  conditions  alone  it  is  extremely  difficult  for  the  student  to 
grasp  these  relations.  The  heart  is  a  very  large  organ.  It  consists  of  two  auricles 
and  a  ventricle  with  two  limbs.  The  auricles  have  thin  walls  and  are  separated 
from  one  another  by  a  very  thin  membrane,  the  septum  superius,  S.s.  The  right 
auricle,  Au.d,  receives  upon  its  dorsal  side  the  opening  of  the  right  vein  or  common 
cardinal,  the  opening  being  guarded  by  valves.  Of  these  valves,  the  one  toward 
the  median  line  disappears,  but  the  other,  toward  the  right  of  the  embryo,  per- 
sists to  form  both  the  Eustachian  and  Thebesian  valves  of  the  adult.  As  stated 
above;  the  left  common  cardinal  delivers  its  blood  to  the  right  vein,  and  so  indi- 
^eart.  The  ventricles  of  the  heart  are  much  larger  than  the  auricles, 
•icular  limb  or  future  left  ventricle,  Ven.S,  is  already  larger  than 
external  groove,  F,  which  marks  the  boundary  between  the 
^vn  by  the  section.  The  trabecular  structure  of  the 
'"fords  a  diagnostic  n^ark  by  which  the  ventricles. 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  283 

if  they  are  cut,  may  be  easily  recognized  in  sections.  The  development  of  the 
trabeculae  corresponds  to  the  formation  of  blood  sinusoids  of  the  heart.  The 
trabeculae  consist  of  young  muscle-cells,  and  each  bundle  of  cells  is  closely  invested 
by  the  endothelium  of  the  heart.  The  blood  thus  circulates  freely  between  the 
trabeculae,  but  remains,  as  in  every  blood-channel,  separated  by  the  endothelium 
from  the  neighboring  tissue.  The  tissues  of  the  heart  are  thus  enabled  to  get 
their  nourishment  from  the  blood  circulating  through  the  organ.  The  sinusoidal 
type  of  circulation  which  we  here  encounter  appears  in  the  heart  of  all  vertebrate 
embryos,  and  is  the  permanent  form  of  circulation  in  the  frog.  In  mammals,  on 
the  other  hand,  although  the  sinusoidal  circulation  is  kept  throughout  life  and 
the  ventricles  always  have  their  trabecular  structure,  yet  we  find,  in  addition,  the 
development  of  a  true  capillary  circulation  to  supplement  the  sinusoidal.  This 
capillary  circulation  is  supplied  by  the  coronary  arteries,  and  develops  compara- 
tively late.  Between  the  auricles  and  the  ventricles  the  heart  is  narrow.  This 
constricted  region  is  known  as  the  auricular  canal.  A  broad  partition,  S.a.c, 
divides  the  cavity  of  the  auricular  canal  into  right  and  left  channels,  forming  open 
vessels  between  the  auricles  and  ventricles.  From  the  lower  edges  of  these  channels 
flaps  of  tissue  project  into  the  ventricles.  The  flaps  are  the  anlages  of  the  atrio- 
ventricular  valves. 

Sections  through  the  Anterior  Limbs  to  Show  the  Brachial  Plexus  (Fig.  197).— 
Figure  197  was  drawn  from  a  single  section,  except  that  the  nerves  in  the  limbs 
represent  a  reconstruction  from  several  adjacent  sections.  The  limb-bud,  A.L, 
projects  freely  from  the  side  of  the  body,  is  covered  by  ectoderm,  EC,  and  filled 
with  a  very  dense  tissue,  the  cells  of  which  show  no  very  clear  histological  dif- 
ferentiation. The  spinal  cord,  Sp.c,  is  fairly  well  advanced  in  its  development 
at  this  level,  and  shows  a  darker,  inner  layer,  Epen,  a  middle  gray  layer,  cm,  and 
an  outer  neuroglia,  Ec.gl.  The  cord  is  completely  surrounded  by  the  developing 
pia  mater,  which  is  quite  thin,  but  highly  vascular.  The  ganglia  are  cut  almost 
symmetrically  on  the  two  sides  and  show  their  dorsal  roots.  The  descending 
trunk  from  each  ganglion  is  joined  by  the  ventral  roots,  V.R,  which  arise  from 
the  ventral  zone  of  the  cord  in  several  bundles  which  unite  about  the  same  time 
with  both  one  another  and  the  dorsal  root  to  form  the  main  nerve-trunk,  N.S, 
which  enters  into  the  formation  of  the  brachial  plexus.  Just  after  the  junction 
of  the  two  roots  the  nerve  gives  off  a  branch  which  runs  obliquely  dorsalward 
into  the  anlage  of  the  dorsal  muscles,  Muse.  This  branch  is,  of  course,  the  dorsal 
ramus.  The  trunk,  N,  which  runs  toward  the  limb  is  the  ventral  ramus.  Below 
the  spinal  cord  is  the  notochord,  Nch,  which  is  completely  surrounded  by  a  dense 
mass  of  mesenchymal  cells,  Vert,  the  anlage  of  an  intervertebral  disc.  Triy^ 
in  the  section  are  the  two  descending  aortas,  Ao,  which  are  "t 
uniting  to  form  the  single  median  dorsal  aort"*.  On  either  si<?* 
of  darker  cells  from  the  aorta  upward  toward  the  later' 
vertebral  disc.  The  dark  cells  belong  to  the  sympathc' 


STUDY  OF  PIG  EMBRYOS. 


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TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  285 

accompanied  by  nerve-fibers.  Below  the  aorta  runs  a  ring  of  epithelium,  (E, 
representing  the  entoderm  of  the  oesophagus,  and  farther  ventralward  a  second 
layer  of  epithelium,  Tra,  the  entodermal  lining  of  the  trachea.  Both  of  these 
rings  of  epithelium  are  surrounded  by  somewhat  condensed  mesenchyma,  the 
differentiation  of  which  about  the  oesophagus  is  more  advanced  than  about  the 
trachea.  Around  the  oesophagus  next  to  the  epithelium  is  a  thin,  looser  layer  of 
mesenchyma,  the  anlage  of  the  mesodermic  portion  of  the  future  mucous  membrane, 
and  perhaps  also  of  the  submucosa.  Outside  of  this  looser  mesenchymal  envelope 
is  a  second  denser  layer  in  which  the  cells  appear  elongated,  having  begun  their 
differentiation  into  smooth  muscle-cells.  To  the  right  and .  left  of  the  oesophagus 
and  at  a  slightly  lower  level  lie  the  sections  of  the  vagus  nerves,  the  right  nerve 
being  situated  a  little  higher  than  the  left.  To  the  right  and  the  left  of  the  aorta 
appear  the  very  large  posterior  cardinal  veins,  card.  From  the  sides  of  the  trachea 
project  lobes  of  tissue  which  represent  the  anlages  of  the  lungs.  These  lobes  of 
tissue  are  each  covered  by  a  layer  of  mesothelium,  and  protrude,  as  it  were,  into 
the  ccelom  of  the  pleural  cavities,  ^leur.  Farther  to  one  side  the  ccelom,  Coe, 
of  the  abdominal  cavity  is  also  in  part  shown.  It  is  bounded  externally  by  the 
body-wall,  Som,  of  the  embryo.  Below  the  trachea  in  the  median  line  is  a  small 
blood-vessel,  a  section  of  the  pulmonary  vein.  As  regards  the  great  nerve  of  the 
limb,  N.S,  it  must  be  remembered  that  it  forms  a  portion  of  the  brachial  plexus 
and  is  joined  by  other  cervical  nerves.  From  the  voluminous  trunk  thus  developed 
there  arise  three  principal  branches;  the  first,  xx,  is  at  the  base  of  the  limb,  is 
small,  and  runs  off  dorsally.  The  other  two  represent  a  terminal  forking  of  the 
nerve-trunk,  one,  yy,  running  to  the  dorsal  side  of  the  limb,  the  other,  zz,  to 
the  ventral  side. 

Section  through  the  Stomach  and  Liver  (Fig.  198). — We  now  pass  to  a  r  section 
well  below  the  heart  in  order  to  study  the  characteristics  of  the  Wolffian  body, 
stomach,  and  liver.  At  this  level,  as  comparison  with,  figures  194  and  196  will 
show,  the  body  of  the  embryo  has  its  greatest  dimensions.  The  upper  edge,  Um, 
of  the  umbilical  cord  appears  in  this  section.  The  spinal  cord  with  its  ganglia 
and  nerves  presents  essentially  the  same  features  as  in  figure  196.  The  notochord, 
Nch,  forms  a  small  circle  in  section  and  is  surrounded  by  the  anlage  of  an  inter- 
vertebral  disc,  which  appears  as  an  area  relatively  large,  over  which  the  mesen- 
chymal cells  are  more  crowded  or  condensed  than  elsewhere.  At  its  peripliery  the 
anlage  merges  without  divisional  boundary  into  the  surrounding  niesencb'  ui. 
It  is  more  expanded  laterally  than  ventrally.  In  the  median  line  belc  noto- 

chord is  the  large  dorsal  aorta,  Ao,  which  is  formed  by  the  union  of  the  two 
descending  aortae  shown  in  figure  196,  and  which  extends  h  the  abdominal 

region  of  the  embryo  to  the  pelvic  region,  where  it  fork-  ;m  the  two  allantoic 

arteries,   which,  passing  on   either  side  of  the  intestine,  Lie  their  course  al<«ig 

the  side  of  the  internal  allantois  or  future  blade1   ,  jiey  reach  the  umbilicus, 

where    they    enter    the    umbilical    cord    to    su^  *  -a -embryonic    or   placental 


286 


STUDY  OF  PIG  EMBRYOS. 


X 


circulation.  The  aorta  is  surrounded  by  mesenchyma,  and  to  this  are,  so  to 
speak,  appended  the  large  Wolffian  bodies,  W.B,  one  on  each  side.  From  the 
dorsal  region  of  the  embryo  to  the  umbilical  cord  extends  the  somatopleure  or 
body-wall,  Som,  which,  like  that  around  the  pericardial  chamber,  consists  of  an 

•Sp.c. 


V.U.D. 


Um 


Om.min. 


G.bl. 


V.U.S 


FIG.  198.  —  PIG,  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  633. 

Ao,  Dorsal  aorta.  EC,  Ectoderm.  G,  Spinal  ganglion.  G.bl,  Gall-bladder.  Gen,  Anlage  of  genital  gland. 
Li,  Liver,  mes,  Somatic  mesenchyma.  msth,  Somatic  mesothelium.  ^Y,  Spinal  nerve.  Nch,  Notochord. 
Om.maj,  Omentum  ma  jus.  Om.min,  Omentum  minus.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  St, 
Stomach.  Um,  Umbilical  cord.  V  '.card,  Posterior  cardinal  vein.  V.C.I,  Vena  cava  inferior.  Vert,  Anlage 
of  w.  '?-;...  V.U.D,  Right  umbilical  vein.  V.U.S,  Left  umbilical  vein.  W.B,  Wolffian  body.  X  22' 
diams. 


external   ectoderm,   Ec^   middle   mesenchyma,    mes,   and    an    internal   mesothelium, 

'•sth.     It  is  important  fur\  the  student  to  understand  the  arrangement  of  the  germ- 

rs  in  the  sonutopleureX    The   mesothelium  is  commonly  known   in   the   descrip- 

:natomy   of    the    :-<Hnlt    ,,s    the   peritoneal   epithelium.     The  peritoneal   membrane 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  287 

V  ' 

consists  of  this  epithelium  and  of  the  underlying  connective  tissue.  In  sections  like 
that  figured  it  can  be  readily  followed  not  only  over  the  inner  surface  of  the  body- 
wall,  but  over  the  surface  of  the  Wolffian  body  and  liver,  and  upon  the  left  side 
of  the  body  also  over  the  surfaces  of  the  greater  omentum,  stomach,  and  lesser 
omentum.  The  relations  of  the  abdominal  viscera  to  the  peritoneum,  which  are 
so  perplexing  to  the  student  of  adult  anatomy,  are  here  shown  diagrammatically, 
as  it  were,  by  the  section  of  the  actual  embryo.  It  is  evident  from  such  a  section 
that  the  abdominal  cavity  (splanchnocele)  is  completely  bounded  by  mesothelium, 
and  that  all  the  abdominal  viscera  are  outside  of  the  cavity.  This  conception, 
which  is  so  important  yet  so  difficult  to  the  student  of  anatomy,  is  easily  mastered 
by  the  study  of  embryonic  relations.  The  Wolffian  body,  W.B,  is  the  fetal  or 
embryonic  kidney,  and  is  also  termed  the  mespnephros  (compare  page  109).  Rela- 
tively to  other  parts,  it  is  much  larger  in  the  pig  than  in  man  or  the  rabbit.  It 
consists  of"  numerous  epithelial  tubules  very  much  contorted  with  blood  spaces 
between  them,  of  glomeruli  which  always  lie  toward  the  median  and  inferior  side 
of  the  organ,  and,  finally,  of  a  single  longitudinal  canal,  the  Wolffian  duct,  into 
which  all  of  the  tubules  open.  The  tubules  are  formed  by  the  cuboidal  epithelium. 
The  glomeruli  resemble  in  their  structure  those  of  the  kidney.  Each  is  a  bunch 
•of  blood-vessels  covered  in  by  a  layer  of  epithelium  which  forms  one  boundary 
of  the  space  into  which'  the  glomerulus  projects.  The  opposite  side  of  the  space 
is  also  bounded  by  epithelium,  which  at  the  stalk  of  the  glomcrv-'us  becomes 
continuous  with  the  covering  of  the  glomerulus  itself,  the  whole  structure  resem- 
bling closely  that  of  a  Malpighian  corpuscle  of  the  true  kidne".  The  space  around 
each  glomerulus  is  really  the  beginning  of  a  Wolffian  tubule  The  spaces  between 
the  tubules  are  almost  entirely  blood-channels,  and  are  lined  by  enddtKeliui. 
for  the  most  part,  is  closely  fitted  against  the  epithelium  of  the  tul  lies.  Ov_ 
ally  a  small  amount  of  mesenchyma  can  be  found  between  the  tubules,  or 
the  tubules  and  the  nearest  endothelium.  We  have,  accordingly,  in  tilts-,-  <>; 
a.  typical  sinusoidal  circulation.  The  blood  spaces  of  the  Wolffian  body  really 
belong  to  the  posterior  cardinal  veins  into  which  the  Wolffian  tubules  in  the  course 
of  their  development  have,  as  it  were,  penetrated,  although  without  destroying  the 
continuity  of  the  vascular  endothelium  It  is  by  the  intercrescencf  of  the  tubules 
and  of  the  endothelium  that  the  sinusoidal  condition  is  establish^- 
the  original  channel  remains  on  the  dorsal  side  of  the  Wolffiar  mu. 

less  free,  V '.card.  We  thus  learn  that,  owing  to  the  developmen  of  ilu-  Wolffian 
body,  the  posterior  cardinal  veins  as  such  disappear.  The  Wolffian  di, 
on  the  ventral  side  of  the  organ,  and  can  easily  be  traced  through  as  a  continuous 
tube  from  section  to  section.  In  the  figure  it  may  be  easily  found  in  the  left 
mesonephros,  it  being  there  the  lowermost  of  the  cavities  drawn  in  the  organ. 
On  the  median  lower  surface  of  the  Wolffian  body,  underneath  the  glomeruli, 
is  an  accumulation  of  tissue.  Gen,  the  anlage  of  the  genital  gland,  which  is  yet 
very  slightly  advanced.  Below  the  "  -n  HUM,  side  of  the  embryo  is  a 


288 


STUDY  OF  PIG  EMBRYOS. 


large  trunk  of  the  vena  cava  inferior,  V.C.I,  on  its  way  past  the  right  dorsal  lobe 
of  the  liver.  Near  the  aorta  on  the  left  is  the  mesogastrium,  Om.maj,  or  future 
great  omentum,  by  which  the  stomach  is  suspended  from  the  median  dorsal  wall 
of  the  abdomen.  The  stomach,  St,  is  entirely  upon  the  left  side  of  the  body  and 
is  directly  connected  with  the  liver  by  means  of  the  anlage  of  the  lesser  omen- 
tum, Om.min.  The  walls  of  the  stomach  are  constituted  by  the  splanchnopleure, 
and,  therefore,  comprise  a  layer  of  thickened  entoderm,  which  bounds  the  cavity 
of  the  organ,  and  a  relatively  thick  layer  of  mesoderm  which  forms  the  greater 

&  f*** 


©  0G>©     e«<%r®     <= 
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Hep 


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Ves.ep 


Msth 


FIG.  199. — SECTION  OF  GALL-BLADDER  OF  A  14.0.1™.  PIG.     FRONTAL  SERIES  67,  SECTION  171.  - 
Hep,  Hepatic  cells.     Mes,  Mesenchyma.     Msth,  Mesothelium.     Ves.ep,  Epithelium  of  gall-bladder. 

t,.ui'  JL  the  wall,  and  the  very  thin  superficial  mesothelium.  The  entoderm  is  a 
smooth  layer  of  moderate  thickness  composed  of  elongated  epithelial  cells.  It 
:orms  no  folds  and  shows  no  trace  of  differentiation  into  gastric  glands.  In  the 
nesenchyma  there  are  some  capillary  blood-vessels.  The  mesothelium  is  thicker 
.han  over  the  liver  and  somatopleure,  and  contains  crowded,  more  or  less  nearly 
>pherical  nuclei.  The  liver,  Li,  is  by  far  the  largesT  organ  of  the  body.  It 
.ip  nearly  half  the  section.  It  is  divided  into  four  main  lobes,  the  two  dorsal  -and 

:ral;   two   on    the   right   and   two   on   the   leu.     The   reference   line.  Li,    : 
ie  left  dorsal   lobe.     The   liver   consists   of   a   complicated   network  of   n 


SAGITTAL  SECTIONS  OF  EMBRYO  OF  12  MM. 


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292  STUDY  OF  PIG  EMBRYOS. 

of  its  various  parts  to  one  another.  The  hind-brain  begins  at  the  spinal  cord, 
Sp.c,  and  has  a  very  large  cavity,  the  fourth  ventricle,  Ven.IV.  It  is  separated  from 
the  region  of  the  mid-brain  by  a  constriction  which  is  very  marked  on  the  dorsal 
side,  Isth.  The  constriction  is  known  as  the  isthmus.  It  is  always  from  the  dor- 
sal side  of  the  isthmus  that  the  fourth  nerve  takes  its  origin.  It  is  one  of  the 
fixed  landmarks  of  the  brain.  The  mid-brain,  M.B,  also  has  a  large  cavity,  and, 
as  a  whole,  forms  the  great  arch  which  corresponds  to  the  head-bend  of  the  em- 
bryo. It  passes  forward  and  downward,  without  any  very  definite  line  of  demarca- 
tion at  this  stage,  into  the  fore-brain,  the  cavity  of  which  is  larger  in  diameter 
than  that  of  the  mid-brain.  The  fore-brain  is  partially  subdivided  into  two  regions; 
the  anterior,  Pros,  is  the  prosencephalon  and  gives  rise  to  the  lateral  outgrowths 
which  form  the  cerebral  hemispheres.  Already  the  deep  depression  separates  this 
part  of  the  fore-brain  on  its  dorsal  side  from  the  posterior  part,  which  is  termed 
the  diencephalon.  The  limits  of  the  diencephalon  at  this  stage  are  very  indistinct; 
later  its  boundary  against  the  mid-brain  becomes  clearly  marked  by  the  differentia- 
tion of  the  epiphysis  and  posterior  commissure.  The  spinal  cord,  Sp.c,  forms  almost 
a  rightv  angle  with  the  axis  of  the  hind-brain.  This  angle  marks  and  corresponds 
to  the  neck-bend  of  the  embryo.  On  its  dorsal  side  the  hind-brain  has  a  thin  epen- 
dymal  roof,  epen,  which,  however,  toward  the  isthmus  thickens  considerably  to 
produce  the  anlage,  Cbl,  of  the  median  portion  of  the  cerebellum.  On  the  ventral 
side  the  wall  of  the  hind-brain  varies  in  appearance.  Where  the  section  is  exactly 
median,  it  displays  the  raphe  or  floor-plate  of  the  region.  Where  it  is  off  the  me- 
dian plane,  it  shows  instead  the  thicker,  lateral  wall  of  the  medulla  oblongata. 
The  walls  of  the  mid-brain  on  the  dorsal  side,  Q,  are  almost  uniform  in  thickness 
and  texture.  They  are,  however,  later  to  be  differentiated  into  the  corpora  quadri- 
gemina.  The  ventral  side  of  the  mid-brain,  Ped,  is  considerably  thicker  than  the 
dorsal,  and  forms  a  strongly  marked  arch.  It  is  represented  in  the  adult  essen- 
tially by  a  part  of  the  peduncle  of  the  cerebrum.  The  floor,  Dien.fl,  of  the  dien- 
cephalon is  a  thin  membrane  of  which  the  part  nearest  to  the  mid-brain  will  pro- 
duce the  mammary  bodies,  and  the  part  farther  from  the  mid-brain  the  tuber 
cinereum.  It  has  already  formed  a  special  outgrowth,  Inf,  the  anlage  of  the 
infundibular  gland,  which  extends  put  from  the  brain  and  arches  over  the  end  of  the 
hypophysis,  Hyp.  The  hypophysis  is  an  outgrowth  from  the  ectodermal  lining  of 
the  mouth,  Or.  Its  method  of  development  can  be  .clearly  made  out  at  this  stage. 
The  infundibular  gland  in  older  embryos  extends  farther  on  the  posterior  side  of 
the  hypophysis.  Meanwhile  the  hypophysis  loses  all  connection  with  the  epithelium 
of  the  oral  cavity,  somewhat  as  does  the  otocyst  with  the  overlying  epidermis 
which  produces  it.  The  hypophysis  proper  and  the  infundibular  gland  undergo 
their  further  development  in  intimate  association.  The  result  of  their  differentia- 
tion is  the  pituitary  body,  which  is  really  a  duplex  organ.  Below  the  infundibular 
gland  the  wall  of  the  brain  shows  a  thickening,  Chi.op,  which  can  be  followed 
through  hi  the  series  laterally  until  it  connects  with  the  optic  stalk.  This  thicken- 


SAGITTAL  SECTIONS  OF  EMBRYO  OF  12  MM.  293 

ing  of  the  brain-wall  in  later  stages  furnishes  the  passage  for  the  fibers  of  the  optic 
nerve,  and  is,  therefore,  the  anlage  of  the  optic  chiasma.  Between  the  infundibular 
gland  and  the  optic  chiasma  extends  the  post-optic  lamina,  L.p.o.  On  the  opposite 
side  of  the  chiasma  follows  the  lamina  terminalis,  which  leads  us  forward  to  the 
wall  of  the  hemispheres,  H.  Underneath  the  hind-brain  extends  the  large  basilar 
artery,  A.bas;  at  its  posterior  end,  A.bas.p,  the  basilar  artery  is  joined  by  the  two 
vertebral  arteries  from  the  fusion  of  which  it  is  really  produced.  Underneath  the 
fore-brain  we  have  the  opening  of  the  mouth,  Or,  from  the  dorsal  side  of  which 
springs  the  elongated  evagination  of  the  hypophysis.  The  oral  cavity  runs  into  the 
pharynx,  the  floor  of  which  is  formed  in  part  by  the  anlage  of  the  tongue,  Ton,  and 
of  the  epiglottis,  Epgl,  a  rounded  eminence  .  very  different  in  shape  at  this  stage 
from  the  adult  epiglottis.  The  pharynx  can  be  followed  along  until  it  passes  over 
into  the  oesophagus,  (E,  which,  however,  is  not  well  shown,  as  the  section  passes 
through  it  away  from  the  true  median  plane.  Between  the  oesophagus  and  the 
anlage  of  the  epiglottis  is  a  mound  of  tissue,  La,  which  represents  the  lateral  wall 
of  the  developing  larynx.  The  mound  is  separated  from  the  anlage  of  the  epiglot- 
tis by  a  deep  notch.  In  the  median  plane  the  mound  is  filled  with  entoderm 
which  forms  a  wide  plate  through  which  there  is  only  a  narrow  opening  leading 
down  into  the  trachea.  Finally,  we  see  from  the  base  of  the  mandible  the  somato- 
pleure,  Som,  extending  off  to  form  the  boundary  of  the  pericardial  chamber.  The 
figure  also  includes  a  presentation  of  the  inferior  maxillary  vein,  V.  mx.i,  and  of  the 
thyroid  gland,  Thyr,  which  immediately  overlies  the  main  trunk  of  the  ventral 
aorta.  This  aorta  gives  off  on  either  side  of  the  pharynx  three  principal  branches, 
of  which  the  smallest  is  the  base  of  the  carotid  and  corresponds  to  the  third  aortic 
.  arch.  The  second  and  third  branches  are  much  larger  and  correspond  to  the  third 
and  fourth  aortic  arches.  The  pulmonary  aorta,  P.Ao,  is  already  separated  from 
the  main  aorta  of  the  body. 

Sagittal  Section  of  the  Head  through  the  Principal  Ganglia  (Fig.  202).— The 
section  is  to  one  side  of  the  median  plane.  It  exhibits  the  optic  nerve,  the  tri- 
geminal,  acustico-facial,  petrosal,  jugular,  and  nodosal  ganglia;  but,  on  the  other 
hand,  exhibits  little  of  the  brain,  there  being  only  a  shaving  from  the  lateral  wall 
of  the  fore-brain,  H,  and  a  section  of  the  widest  part  of  the  hind-brain  which 
shows  the  cavity  or  lateral  recess,  R.L,  of  the  fourth  ventricle.  The  auditory  vesicle 
is  cut,  Ot.  It  is  formed  by  a  layer  of  epithelium  derived  from  the  ectoderm, 
although  now  not  connected  with  the  overlying  part  of  the  epidermis  by  the  in- 
vagination  of  which  the  octocyst  is  developed.  It  shows  a  narrow,  upward  pro- 
longation, the  anlage  of  the  ductus  endolymphaticus  (compare  Fig.  42).  The 
epithelial  otocyst  lies  in  a  line  with  the  great  cephalic  ganglia  and  occupies  its 
invariable  and  permanent  position  behind  the  acustico-facial  ganglion,  Ac.F,  and  in 
front  of  the  glosso-pharyngeal,  G.petr.  The  position  of  the  otocyst  makes  it  an 
invaluable  landmark  in  the  stud  rtions  of  the  head.  Only  the  lateral  no 

of  the   pharynx.  Ph,   appears.     It   forms  a  \vicl<  it    slit-like  diverticulum. 


294 


STUDY  OF  PIG  EMBRYOS. 


which  extend  farther  laterally  the  first  and  second  entodermal  gill-pouches.  In 
the  figure  can  be  seen  a  small  depression  extending  downward  from  the  cesophageal 
or  posterior  end  of  the  pharynx.  This  depression  marks  the  beginning  of  the  second 
cleft.  Nothing  is  seen  of  the  third  and  fourth  clefts  in  this  section,  as  they  both 
lie  nearer  the  median  plane.  The  pocket  or  diverticulum  of  the  cervical  sinus, 


Jug.'"       G.jug. 


Ph. 


Ac.F. 


R.L. 


EC. 


G.petr. 


G.nod. 


Cerv.S. 


N.12. 


Cerv.f). 


Jug."" 


FIG.  202.— PIG,  12.0  MM.     No.  7.     SAGITTAL  SECTION  25. 

Ac.F,  Acustico-facial  ganglion  complex.  Aur,  Auricle  of  the  heart.  Cerv.S,  Diverticulum  of  the  cervical  sinus* 
just  in  front  of  which  shows  the  anlage  of  the  thymus,  which  is  deeply  stained.  Cerv.  6,  Sixth  cervical  nerve- 
Cce,  Ccelom  around  the  heart  or  pericardial  cavity.  EC,  Ectoderm.  G.jug,  Ganglion  jugulare  of  the  vagus 
nerve.  G.nod,  Ganglion  nodosum  of  the  vagus  nerve.  G.petr,  -Ganglion  petrosum  of  the  glosso-pharyngeal 
nerve.  G.tri',  Ganglion  of  the  trigeminus  nerve.  H,  Lateral  wall  of  the  cerebral  hemisphere.  Jug'  -Jug'"' , 
Jugular  vein  (Jug',  Behind  the  trigeminus.  Jug" ,  Branch  in  front  of  the  trigeminus.  Jug'" ,  Main  stem 
behind  the  vagus.  Jug"",  Main  stem  descending  to  join  the  duct  of  Cuvier).  m,  An  undetermined 
structure,  probably  the  anlage  of  a  lingual  muscle.  Md,  Mandible.  JV-5,  Root  of  the  fifth  or  trigeminal  nerve. 
N.op,  Optic  nerve.  N.  12,  Twelfth  or  hypcglossal  nerve.  Ot,  Otocyst.  PA.  Pharynx.  R.L,  Recessus  lateralis 
of  the  fourth  ventricle.  Ve,  Small  branch  of  the  jugular  vein.  Vent,  Ventricle  of  the  heart.  X  22  diams. 

Cerv.S,  lies  near  the  ganglion  nodosum,  G.nod.  From  its  appearance  it  might  easily 
be  mistaken  for  the  section  of  a  gill-cleft,  but  it  is  in  reality  lined  not  by  entoderm 
but  by  ectoderm,  and  its  cavity  can  be  easily  traced  through  the  series  of  sections 
of  the  exterior  of  the  embryo  where  the  epithelium  lining  the  sinus  becomes  con- 
tinuous with  the  epidermis.  Cfephalad  from  the  sinus,  but  close  to  it,  lies  a  small 


SAGITTAL  SECTIONS  OF  EMBRYO  OF  12  MM.  295 

dark  rounded  mass,  the  anlage  of  the  nodulus  thymicus  (compare  Fig.  194,  Nod}. 
The  nodulus  anlage  is  produced  by  proliferation  of  the  entodermal  cells  on  the 
anterior  side  of  the  third  cleft,  and  is  penetrated  by  blood-vessels  which  seem  to 
be  sinusoids,  although  their  history  has  not  been  worked  out.  The  great  vein  of 
the  head,  which  for  convenience  we  may  term  the  jugular, — although  the  applica- 
tion of  this  name  to  the  vein  in  its  present  condition  is  somewhat  inexact, — is  cut 
several  times,  owing  to  its  irregular  course.  Its  main  stem,  Jug"" ,  arises  nearly 
vertically  through  the  cervical  region  and  is,  relatively  to  the  size  of  the  embryo, 
of  huge  diameter.  It  continues  upward,  Jug'",  along  the  dorsal  side  of  the  vagus 
to  about  half-way  between  the  ganglion  nodosum  and  ganglion  jugulare.  At  that 
point  the  vessel  curves  inward  and  forward,  and  therefore  is  not  encountered 
again  in  this  section  until,  having  bent  upward  again,  it  shows,  Jug',  on  its  way 
past  the  trigeminal  ganglion.  A  branch  of  the  jugular,  Jug",  is  cut  just  above  the 
ganglion,  and  another  small  and  probably  not  very  important  branch  is  shown 
at  Ve. 

The  nerves  are  shown  as  follows:  The  optic  nerve,  N.op,  still  has  its  central 
cavity,  which,  nearer  the  median  plane,  opens  into  the  third,  ventricle  of  the  brain, 
and  in  the  section  resembles  in  shape  an  inverted  U.  On  the  side  of  the  nerve 
toward  the  mouth  there  is  a  deep  notch — the  section  of  the  choroid  fissure.  The 
trigeminal  ganglion,  G.tri,  .is  very  large,  and  its  trilobate  form  is  clearly  indicated 
by  the  figure.  The  lobe  to  which  the  reference  line,  G.tri,  runs  gives  off  the  ramus 
ophthalmicus;  the  lobe  nearest  the  jugular  gives  off  the  ramus  maxillaris  inferior, 
while  the  middle  lobe  gives  off  the  ramus  maxillaris  superior.  From  the  ganglion 
the  fibers  and  nerve-cells  extend  upward  to  form  the  root,  AT. 5,  which  joins  the 
hind-brain  at  a  characteristic  point — namely,  at  the  summit  of  the  Varolian  bend 
and  where  the  hind-brain  is  widest  (compare  Figs.  189  and  203).  By  its  great 
size  and  by  its  topographical  association  with  the  lateral  apex  of  the  recessus  lateralis 
of  the  fourth  ventricle,  the  trigeminal  ganglion  may  always  be  readily  identified 
in  sections  of  embryos.  The  acustico-facial  ganglia,  Ac.F,  may  also  be  readily 
determined  by  their  typical  position  immediately  in  front  of  the  otocyst,  Ot.  But 
it  is  quite  difficult  to  identify  the  four  components  of  this  complex  structure; 
namely,  i°,  the  motor  root  of  the  facial  nerve;  2°,  the  facial  or  geniculate  ganglion; 
3°,  the  vestibular  ganglion;  4°,  the  cochlear  ganglion.  In  figure  202  three  divisions 
are  shown.  The  large,  darkly  stained  division,  to  which  the  reference  line,  Ac.F, 
runs  and  which  lies  nearest  to  the  otocyst,  is  the  vestibular  portion  of  the  acous- 
tic ganglion;  the  small,  light  area  occupying  a  middle  position  in  the  inferior  part 
of  the  complex  is  the  motor  division  of  the  seventh  nerve,  or  lateral  root  of  the 
facial;  it  can  be  followed  to  the  brain,  which  it  enters  as  four  bundles  of  fibers; 
its  path  of  entrance  is  shown  better  in  frontal  sections  (Fig.  204,  t.m).  Just  in 
front  of  the  facial  motor  root  lies  a  second  smaller  dark  mass,  the  geniculate  gan- 
glion of  the  facial,  with  an  upward  prolongation,  the  sensory  root.  The  ninth  or 
glosso-pharyngeal  nerve  is  represented  by  the  ganglion  petrosum,  G.petr,  and  its 


296  STUDY  OF  PIG  EMBRYOS. 

ascending  sensory  root.  This  nerve  may  be  quickly  identified  because  it  is  the  first 
behind  the  otocyst.  The  upper  ganglion  of  this  nerve,  the  so-called  Ehrenritter 's 
ganglion,  is  represented  by  an  accumulation  of  cells  in  the  upper  part  of  this 
root.  As  regards  the  tenth  nerve,  or  vagus,  both  its  ganglia  and  the  fibrous  trunk 
connecting  them  are  shown.  The  upper  or  jugular  ganglion,  G.jug,  is  nearly  on 
a  level  with  the  otocyst,  while  the  lower  or  nodosal  ganglion,  G.nod,  lies  near  the 
cervical  sinus.  To  the  nerve-trunk  between  the  two  ganglia  are  adjoined  the  fibers 
of  the  eleventh  or  spinal  accessory  nerve,  which  does  not  otherwise  appear  in  this 
section.  A  small  piece  only  of  the  hypoglossal  nerve  can  be  seen,  N.I2.  The 
space  occupied  by  this  nerve  is  blank  in  the  engraving;  in  the  specimen  it  shows 
horizontal  fibers. 

Pig  Embryo  of  12.0  mm.     Study  of  Frontal  Sections. 

The  frontal  series  has  special  value  for  the  study  of  the  hind-brain  and  asso- 
ciated structures,  as  the  plane  of  the  section  is  approximately  at  right  angles  to 
the  axis  of  the  hind-brain.  It  also  furnishes  instructive  pictures  of  the  vena  cava 
inferior  and  of  the  relations  of  developing  vertebrae  and  nerves. 

Portions  of  three  sections  illustrating  the  structure  of  the  hind-brain  and  asso- 
ciated parts  are  given  below.  The  following  remarks  on  the  hind-brain  are  in- 
tended to  make  clearer  the  significance  of  these  sections.  The  wall  of  the  hind- 
brain  is,  of  course,  produced  by  the  development  of  the  wall  of  the  medullary 
tube.  Its  most  striking  peculiarity  is  the  enormous  expansion  of  the  deck-plate, 
which  forms  the  very  wide  epithelial  layer,  (Fig.  203,  epen),  the  so-called  ependymal 
roof  of  the  fourth  ventricle.  It  starts  from  the  upper  edge  of  the  dorsal  zone, 
D.Z,  and  forms  a  wide  arch  which  is  covered  in  externally,  by  a  rather  thin  layer 
of  mesoderm,  mes,  and  the  nearby  epidermis,  EC,  of  the  embryo.  The  covering 
is  so  ^slight  in  development  at  this  stage  that  in  the  fresh  specimen  the  roof  of 
the  fourth  ventricle,  including  its  coverings,  appears  as  a  translucent  membrane 
through  which  we  can  readily  distinguish  the  great  cavity  of  the  fourth  ventricle 
itself.  The  expanse  of  the  ependymal  arch  is  greatest  at  the  region  of  the  tri- 
geminal  root.  From  there  backward  toward  the  spinal  cord  its  expanse  gradually 
diminishes.  In  correspondence  with  the  growth  of  the  deck-plate  the  lateral  walls 
of  the  medullary  tube  become  bent  outward  and  downward,  so  that,  though  they 
remain  near  together  on  their  ventral  side,  where  they  are  united  by  the  floor-plate 
or  median  raphe  (Fig.  205,  raph),  yet  their  upper  dorsal  edges  are  far  apart.  In 
consequence  of  this  change  of  their  position  the  original  lateral  walls  appear  as 
the  floor  of  the  hind-brain,  and  we  recognize  in'  them  the  anlages  of  the  medulla 
oblongata.  We  distinguish  here,  as  everywhere  in  the  medullary  wall,  the  dorsal 
and  ventral  zones.  The  ventral  zone  is  intimately  'united  with  its  fellow  by  the 
short  median  raphe.  Between  them  is  a  deep  fissure  (Fig.  204.  /)  ,which  is  never 
wholly  obliterated.  The  floor-plate  undergoes  a  great  development  in  later  stages 
and  is  transformed  into  the  median  raphe  of  the  adult  medulla.  The  lateral  or 


FRONTAL  SECTIONS  OF  EMBRYO  OF  12  MM. 


297 


morphologically  dorsal  limit  of  the  ventral  zone  is  marked  by  the  exit  of  the  lateral 
roots  (Fig.  203,  L.R).  The  ventral  limit  of  the  dorsal  zone  is  marked  by  the  en- 
trance of  the  sensory  or  ganglionic  fibers  (Fig.  203,  G.tri;  Fig.  204,  Fac}.  Toward 
the  dorsal  side  the  dorsal  zone  gradually  thins  out  and  passes  over  into  the 
ependyma,  epen.  The  great  development  of  the  lateral  roots  is  perhaps  the  most 
important  single  characteristic  of  the  medulla  oblongata.  They  furnish  the  principal 
motor  or  efferent  nerve-tracts  of  the  brain  and  form 
an  important  constituent  part  of  four  nerves:  first, 
the  trigeminal  or  fifth;  second,  the  facial  or  seventh; 
third,  the  glosso-pharyngeal  or  ninth;  and  fourth, 
the  vagus  or  tenth.  There  are  no  lateral  roots 
known  to  occur  anterior  to  the  medulla  oblongata, 
unless  possibly  the  fourth  nerve,  the  relations  of 
which  in  many  respects  are  peculiar,  should  turn 
out  to  be  a  lateral  root.  In  the  spinal  cord  we 
find  lateral  roots  in  the  upper  cervical  region,  and 
it  is  not  improbable  that  they  may  yet  be  found 
associated  with  the  dorsal  roots  of  spinal  nerves 

lower  f'down.      But   even    in    the    cervical    cord    the 

• 

lateral  roots  attain  but  a  slight  development.      The      Card 
contrast  with  other  portions  of  the  central  nervous 
system   makes   the  great  development  of  the  lateral 
roots   in   the   medulla  oblongata  all  the  more  strik- 
ing.    The   dorsal   zone   of  the  hind-brain  lags  con- 
siderably   behind    the    ventral    zone   in   its   develop-     FIG.    203 
ment,    and    at    all   stages  the   ventral  zone  forms  a 


PIG,     12.0     MM.    FRONTAL 
SERIES  6,  SECTION  284. 


larger    proportion    of    the    medulla    th  n    does   the     Card>  Anterior  cardinal  vein-    D-Z'  UP~ 

per  portion  of  the  dorsal  zone  of  His. 
EC,     Ectoderm,     epen,     Ependymal 


roof  of  the  fourth  ventricle.  G.tri, 
Ganglion  trigemini.  L.R,  Lateral 
root  of  the  trigeminal  nerve,  mes, 
Mesenchyma.  T.S,  Tractus  soji- 
tarius  of  W.  His.  X  22  diams. 


Section  through  the  Trigeminal  Roots  (Fig.  203). 
—The  section  passes  through  the  widest  part  of  the 
hind-brain,   the  cavity  of  which  is  enormously  dis- 
tended.     It   is    bounded    on    the    dorsal    side    only 
by    the    very    thin    ependymal    roof,    epen,    which 

does  not  form  any  part  of  the  true  nervous  structure,  although  it  passes  into  and 
is  directly  continuous  with  the  dorsal  zone,  D.Z,  which  is  thus  seen  to  be  only  a 
thickened  portion  of  the  wall  of  the  neural  tube,  just  as  the  ependyma  is  the 
attenuated  deck-plate.  The  trigeminal  ganglion,  G.tri,  is  very  large  and  sends 
its  sensory  fibers  upward  into  the  dorsal  zone  to  form  there  a  distinct  bundle  of 
nerve-fibers  which  persists  throughout  life  and  is  known  in  the  adult  as  the  tri- 
geminal tract,  T.S.  The  entering  sensory  fibers  fork;  their  ascending  branches  form 
the  relatively  short  ascending  tract,  their  descending  branches  the  much  longer 
descending  tract,  which  gradually  grows  through  the  length  of  the  medulla  oblon- 


298 


STUDY  OF  PIG  EMBRYOS. 


gata   well    outside    the    tractus    solitarius,    which,    however,    it   joins   just   before    the 
spinal  cord  is  reached.     The  other  root  of  the  nerve,  L.R,  is  lateral.     It  lies  below N 
the  ganglion  near  the  median  plane.     Its  fibers  arise  from  neuroblasts  in  the  ventral 
zone   and   gather  together  as   a  distinct  bundle   which   starts   near   the   median   line, 
takes    a    curving    course    through    the    ventral    zone,    and    makes    its    exit    from   the 

medullary  wall  at  the  dorsal  limit  of  the 
zone.  It  has  a  striking  resemblance  to  the 
root  of  the  facial  nerve.  We  do  not  yet 
know  whether  such  a  course  of  the  fibers 
is  characteristic  of  all  lateral  roots  or  only 
of  the  trigeminal  and  facial  roots.  On  the 
medial  side  of  the  trigeminal  ganglion  is  a 
large  vein,  Card,  the  anterior  cardinal  vein. 
In  the  median  line  in  the  mesenchyma 
immediately  below  the  raphe  is  the  section 
of  the  basilar  artery,  and  considerably  below 
that  is  the  small  section  of  the  notochord 
which  it  is  very  difficult  to  distinguish  with 
a  low  power.  Between  the  notochord  and 
the  cardinal  vein  is  the  section  of  the 
carotid  artery. 

m     Section      through       the       Acustico-facial 
FIG.    204.— PIG,    12.0   MM.    FRONTAL   SERIES   6,      Ganglion    (Fig.     204).— In    this    section    the 


SECTION  340. 

A. has,  Arteria  basilaris.  D.Z,  Dorsal  zone  of  the 
medulla  oblongata.  EC,  Ectoderm,  epen, 
Ependymal  roof  of  the  fourth  ventricle.  /, 


thickened  ventral  wall  of  the  hind-brain 
(i.  e.,  the  anlage  of  the  medulla  oblongata) 
is  not  spread  out  nearly  horizontally,  as  in 


N.I2,  Hypoglossal  nerve.     PA,  Pharynx,     t.m, 
Motor  tract  of  facial  nerve.      X  22  diams. 


Median  fissure  of  the  medulla  oblongata.    Fac,     the  trigeminal  region,  but  rises  obliquely  on 
Sensory  root  of  the  facial  nerve.    G.gen,  Gen-     either    side    from    the    median    line.      The 

iculate  ganglion   of  the  facial   nerve.     G.vest,         •    ,  .  j      i    r,        •  i  r      .1  j    n 

.    ,  ,  right    and    left    sides    of    the    medulla    are 

Ganglion  vestibuli  of  the  acoustic  nerve.     Jug,  • 

Lateral  vein,    mes,  Mesenchyma.    Mx.i,  In-     divided     from     one     another     by     a    deep 

ferior  maxillary  branch  of  the  trigeminal  nerve.       median    fissure,    /.        In    the    median    line    WC 

see  also  the  basilar  artery,  A.bas,  and 
still  lower  the  wide,  slit-like  pharynx,  Ph, 
the  outer  portion  of  which  ascends  obliquely  toward  the  lateral  vein,  Jug.  The 
ascending  lateral  part  of  the  pharynx  is  a  portion  of  the  first  gill-pouch  or  future 
Eustachian  tube,  and  is  quite  clearly  marked  off  from  the  pharynx  proper  by 
its  oblique  direction.  Of  the  acustico-facial  ganglion  complex  the  section  shows 
four  parts:  the  ganglion  vestibuli,  G.vest;  the  geniculate  ganglion,  G.gen;  the  sensory 
root,  Fac,  of  the  facial  nerve  arising  from  the  geniculate  ganglion  and  entering 
the  brain  to  form  there  a  distinct  fiber-tract  which  is  oval  in  the  section  and  lies 
just  below  the  entering  vestibular  fibers,  and  is  clearly  indicated  in  the  drawing; 
and,  finally,  the  motor  tract,  t.m,  of  the  facial  nerve.  This  tract  is  a  very  dis- 


'  FRONTAL  SECTIONS  OF  EMBRYO  OF  12  MM. 


299 


epen 


tinctly  marked  bundle  of  nerve-fibers  which  arise  from  neuroblasts  of  the  ventral 
zone,  traverse  that  zone  almost  horizontally,  then  bend  downward  and  pass  out 
from  the  brain-wall,  appearing  as  the  lateral  root  of  the  facial  nerve.  The  root 
runs  first  toward  and  then  past  the  geniculate  ganglion.  The  cardinal  vein  origi- 
nally was  inside  the  ganglia;  by  island  formation  it  has  migrated  outside  the  ganglia, 
forming  the  lateral  vein,  Jug.  In  the  mandible  below  the  pharynx  appear  two 
nerves.  Of  these,  the  upper  is  the  hypo- 
glossal,  N.I 2,  which  lies  near  the  angle 
formed  by  the  junction  of  the  first  gill-cleft 
with  the  pharynx.  The  lower  of  the  two 
nerves,  Mx.i,  is  the  inferior  maxillary. 

Section  through  the  Otocyst  (Fig.  205).— 
The  figure  is  from  a  section  not  far  from 
the  last.  The  hind-brain  has  narrowed 
considerably;  its  thickened  floor,  Md.obl,  the 
anlage  of  the  medulla  oblongata,  rises 
steeply  from  the  median  line.  Its  ependymal 
roof,  epen,  is  less  expanded  than  in  figures  Fac.m. 
204  and  205.  It  forms  a  sharp  angle  in 
the  *  dorsal  median  line.  The  median  ven- 
tral fissure  between  the  two  sides  of  the 
medulla  is  deeper  than  farther  forward. 
The  pharynx,  Ph,  is  wide  and  has  expanded 
laterally  into  the  common  beginning  of  the 
first  and  second  gill-pouches.  Between  the  FIG.  205.— PIG,  12.0  MM.  FRONTAL  SERIES  '6, 
pharynx  and  the  raphe  the  basilar  artery,  SECTION  380. 

A.bas,     his    been     CUt     transversely.        Below       ^«,  Basilar  artery.     Coch,  Cochlea.     Z>.e,Ductus 

endolymphaticus.     epen,     Ependyma.     Fac.m, 


D.e. 


S.c. 


Jug. 


Coch. 


Ph. 


Motor  division  of  the  facial  nerve.  Jug,  Vena 
lateralis  capitis.  Md.obl,  Medulla  oblongata. 
Ph,  Pharynx,  raph,  Median  raphe  of  the 
medulla  oblongata.  S.c,  Anlage  of  the  semi- 
circular canals.  Ve,  Vein.  X  22  diams. 


it  and  near  the  pharynx  is  the  small 
notochord,  which,  however,  can  b.e  clearly 
recognized  only  with  the  higher  power, 
and  is,  therefore,  not  represented  in  this  or 
the  preceding  figure.  The  otocyst  is  a  large 
epithelial  vesicle  with  three  well-marked  divisions:  First,  the  common  chamber,  S.c, 
out  of  which  the  three  semicircular  canals  are  to  be  differentiated.  Second,  a 
slender  canal,  D.e.,  which  one  easily  -identifies  as  the  anlage  of  the  ductus  en- 
dolymphaticus. It  lies  between  the  semicircular  canal  and  the  wall  of  the  me- 
dulla oblongata.  Third,  the  long,  curving,  but  not  spiral  cochlea,  Coch.  The  com- 
mon chamber  formed  by  the  union  of  these  divisions  is  later  subdivided  to  form 
the  upper  utriculus  and  lower  sacculus.  Outside  the  cochlea  lies  the  cross-section 
of  the  vena  'lateralis  capitis,  Jug,  which  appears  in  the  adult  as  part  of  the  inter- 
nal jugular.  Just  below  the  lateral  vein  is  the  section  of  the  motor  portion,  Fac.m, 
of  the  facial  nerve.  The  sensory  portion  of  the  facial  nerve  at  this  stage  is 


300 


STUDY  OF  PIG  EMBRYOS. 


much  smaller,  and  runs  only  a  short  distance  downward  from  the  geniculate  gan- 
glion and  is  entirely  separate  from  the  motor  portion.  The  morphological  constitu- 
tion of  the  facial  nerve  is  still  very  obscure,  and  a  satisfactory  account  of  its 
development  is,  for  the  present,  impossible. 

Section    through    the    Vena   Cava  Inferior    (Fig.    206). — The   section   displays   the 
huge    vena    cava    inferior   cut    through    most    of    its  length.     For    the    composition  of 


Ao.n 


Ao.S 


P.A. 


V.s.c. 


'Ao 


FIG.  206. — PIG  of  12.0  MM.     FRONTAL  SERIES  6,  SECTION  423. 

Ao,  Main  dorsal  aorta.  Ao.D,  Right  descending  aorta.  Ao.S,  Left  descending  aorta.  Au.D,  Right  auricle. 
Au.S,  Left  auricle,  c,  Vena  cava  passing  through  the  caval  ligament.  Nch,  Notochord.  Om.min,  Omentum 
minus.  P.A,  Pulmonary  aorta.  Sp.e,  Spinal  cord.  St,  Stomach.  S.V,  Sinus  venosus.  V.C.I,  Vena  cava 
inferior.  V.E,  Valvula  Eustachii.  V.S.  Valvula  sinistra.  V.s.c,  Vena  subcardinalis.  W.B,  Wolffian  body. 
X  15  diams. 

the  vein  see  page  257.  In  the  section  it  starts  between  the  Wolffian  bodies,  W.B,  as 
a  large  vessel,  V.C.I,  formed  by  the  median  union  of  the  two  subcardinal  veins 
of  the  Wolffian  bodies.  It  passes  upward,  c,  through  a  thin  band  of  tissue,  the 
caval  ligament,  to  the  right  of  the  lesser  omentum,  Om.min,  into  the  substance  of 
the  liver,  Li,  through  which  it  takes  a  slightly  sinuous  course.  Several  junctions 


FRONTAL  SECTIONS  OF  EMBRYO  OF  12  MM.  301 

of  the  hepatic  veins  with  the  main  vessel  are  cut  in  the  section.  The  liver  is 
attached  to  the  diaphragm.  Above  the  diaphragm  the  cava  is  continued,  with  thin 
walls,  for  a  short  stretch,  S.V,  which  is  the  modified  sinus  venosus  of  the  heacrt, 
and  which  opens  directly  into  the  right  auricle,  Au.D.  The  opening  is  guarded 
by  two  valves,  the  valvula  sinistra,  V.S,  on  the  left,  and  the  valvula  Eustachii, 
V.E,  on  the  right,  which  together  prevent  the  back-flow  of  the  blood  !rom~THer 
heart  into  the  vein.  Above  the  heart  appear  the  pulmonary  aorta,  P. A,  and  the 
two  descending  aortae,  Ao.D,  Ao.S.  The  main  dorsal  aorta,  Ao,  shows  in  the 
lower  part  of  the  section.  The  stomach,  St,  lies  on  .the  left  side  and  is  closely 
attached  to  the  liver  by  the  short  and  thick  anlage  of  the  great  omentum,  and  is 
attached  to  the  caval  ligament  .by  the  longer  band  of  the  lesser  omentum,  Om. 
min.  The  space  bounded  by  the  stomach,  the  lesser  omentum,  and  the  liver  is 
the  lesser  peritoneal  cavity  (bursa  omentalis).  In  the  Wolffian  body  the  sub-cardinal 
vein,  V.s.c,  is  easily  identified,  and  with  a  higher  power  the  intertubular  sinu- 
soids reveal  their  characteristics  clearly,  the  sinusoidal  epithelium  being  fitted  closely 
to  the  surface  of  the  Wolffian  tubules.  The  division  of  the  ccelom  by  the  dia- 
phragm into  an  upper  pericardial  and  a  lower  abdominal  chamber  is  perfectly  demon- 
strated by  this  section.  The  student  should  observe  that  the  mesothelium  forms 
for  both  chambers  the  absolutely  unbroken  boundary  of  the  ccelom. 

Section  through  the  Dorsal  Vertebra  (Fig.  207). — Owing  to  the  curvature  of  the 
embryo  the  spinal  cord  is  cut  twice;  once,  Sp.c',  toward  the  head  end  of  the 
embryo,  and  again,  Sp.c",  lower  down  toward  the  tail  end.  Alongside  the  sections 
of  the  spinal  cord  appear  the  large,  darkly  stained  masses  of  the  ganglia,  G.  The 
section  also  passes  through  the  bases  of  the  anterior  limbs,  A.L,  in  one  of  whi 
can  see  one  of  the  branches,  N.br,  of  the  brachial  plexus.  Between  the  wo  | 
of  the  spinal  cord  of  the  section  the  plane  passes  on  the  ventral  side  of 
cord  and  shows  the  series  of  vertebral  formations,  together  with  the  nc 
N',N",-  the  intersegmental  arteries,  A.i.s,  and  the  segmental  veins,  small  \essds 
which  lie  close  to  the  intersegmental  arteries.  The  nerves  are  sections  of  the 
dorsal  root  below  the  ganglia.  Each  nerve  has  a  distinct  outline  and  is  partly 
penetrated  by  ingrowing  mesenchymal  cells  which  subdivide  the  nerve  into  rounded 
fiber  bundles.  In  each  bundle  the  nerve-fibers  appear  as  fine  dots,  which,  how- 
ever, by  the  use  of  the  fine  adjustment  can  be  followed  up  and  down  through 
the  section,  and  thus  identified  as  fibers.  The  single  fibers  are  more  or 
less  isolated  from  one  another,  and  between  them  are  delicate  threads,  the  nature 
of  which  is  not  known.  Between  the  adjacent  rounded  bundles  of  fibers  there 
is  often  a  distinct  space.  The  anlages  of  the  intervertebral  disc,  fv.D,  are 
formed  entirely  from  condensed  mesenchyma,  and  therefore  stand  out  somewhat 
conspicuously  in  the  section  owing  to  their  darker  staining.  Each  anlage  is  bow- 
shaped,  the  concavity  of  the  bow  facing  toward  the  tail  of  the  embryo.  The  end^ 
of  the  bow  pass  behind  the  nerve-trunk  of  the  segment  to  which  the  anlage  be- 
longs. The  anlages  extend  completely  across  the  median  line,  and  by  following 


302 


STUDY  OF  PIG  EMBRYOS. 


Mes. 


FIG.  207. — PIG,  12.0  MM.    FRONTAL  SERIES  6,  SECTION  572. 

Inlersegmental  artery.  A.L,  Anterior  limb,  tin,  Cinerea  of  spinal  cord.  EC,  Ectoderm.  G,  Ganglion. 
Iv.D,  Intervertebral  disc.  Mes,  Mesoderm.  N',N",  Nerves.  N.br,  Nerve-branch  of  brachial  plexus. 
Sp.cf,  Cephalad  portion  of  spinal  cord.  -Sp.c".  Caudad  portion  of  spinal  cord.  V.arch,  Anlage  of  arch  of 
vertebra.  X  22  diams. 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  17  MM. 


303 


through  in  the  series  of  sections,  it  may  be  found  that  the  condensed  mesenchyma 
surrounds  the  notochord,  which,  therefore,  passes  through  the  central  portion  of 
each  intervertebral  anlage.  The  bodies  of  the  vertebrae  at  this  stage  consist  merely 
of  the  loose  mesenchyma  between  the  intervertebral  discs,  are  entirely  without  any 
distinct  limitation,  and  merge  into  the  surrounding  loose  mesenchyma.  Near  the 
anterior  border  of  each  nerve-trunk,  and  usually  somewhat  toward  the  median 
side  of  it,  lie  -the  intersegmental  vessels,  which  are  of  small  size  and  vary  greatly 
in  their  exact  position  and  number,  according  as  they  are  more  or  less  branched. 
Between  the  ends  of  the  vertebral  bows  outside  of  the  nerve-trunks  can  be  seen 
with  higher  power  clusters  of  elongated  cells  with  developing  muscle-fibers  which 
are  here  still  segmentally  arranged  between  the  processes  of  the  developing 
vertebrae. 

Pig  Embryo  of  17  mm.     Study  of  Sections. 

Since  the  pig  of  12  mm.  contains  the  anlages  of  perhaps  every  important 
part  of  the  body  sufficiently  advanced  in  development  to  be  clearly  recognized, 
we  find  in  the  immediate  subsequent  development  that  we  have  to  do  not  so 
much  with  an  introduction  of  new  parts  as  with  the  differentiation  of  those  which 
have  already  commenced.  Embryos  of  17  mm.  are  convenient  for  the  study  of 
the  differentiations  referred  to.  Particularly  important  for  the  student  to  note 
are  the  advances  in  the  development  of  the  vertebrae,  of  the  lungs,  of  the  Wolffian- 
bodies  and  genital  glands,  and  of  the  kidneys.  These  points  are  illustrated  in 
figures  208  to  210,  representing  portions  of  three  transverse  sections  of  a  17  mm. 
embryo. 

Transverse  Section  through  the  Lungs   (Fig.   208).  —  The  epidermis  of  the  embryo 
%as  become  more  distinct  owing  to  its  growth  in   thickness,   which  is  •    .ompii.  hed 
by  the  increase  of  the  number  of  layers  of  cells.     The  growth  is  very  marked  al 
the  sides  of  the  section  about  the  level  of  the  vertebra.     At  these  points  it  ca 
early  seen  that  upon  the  outside  the  epidermis  has  a  very  thin  layer  of  flattened 
the    nuclei    of    which    are    themselves    also    somewhat    flattened.     This    single 
jf   cells    is   known    as    the   epitrichium,    because    the    hairs   are    developed  •  en- 
rneath   it.     Where    the    epidermis   is   thickest,    one   can   observe   that   the 


layers 
They 
Between 
forming    the    i 
developed   and 
carries    the    nerves 
of  more  darkly  staine 
cells  proper,  the  anlage 
thelial    muscle-cells, 


to  the  mesoderm  are  closely  packed  together  with  round  nuclei. 
commencing  formation  of  the  basal  layer  of  the  adult  epidermis. 
layer   and    the    epitrichium    the   cells    are   more    loosely    placed, 
of    the    mucous    layer.     The    mesenchyma    is    very    much 
a   large   territory   in    the   dorsal   region   of   the   embryo.     It 
blood-vessels    and    shows    at    various    points    accumulations 
are  of  two  kinds:  first,  groups  of  mesenchymal 
the  skeleton;  and,  second,  groups  of  meso- 
skeletal    muscles.     There    is    little 


differentiation  otherwise 


may  note  the  following  changes 


304 


STUDY  OF  PIG  EMBRYOS. 


in  it:  (i)  The  anlage  of  the  vertebra,  Vert,  which  is  now  quite  well  denned;  around 
the  edge  of  it  the  cells  have  assumed  an  elongated  form  and  have  elongated  nuclei; 
the  elongation  is  parallel  with  the  surface  of  the  anlage.  These  cells  result  from 
the  commencing  differentiation  of  the  perichondrium,  which  at  this  stage  merges 
on  the  one  side  into  the  anlage  of  the  vertebrae,  and  on  the  other  into  the  sur- 
rounding mesenchyma.  The  cells  of  the  vertebra  have  changed  into  young  car- 
tilage-cells. They  are  now  distinctly  separated  from  one  another  by  a  well-devel- 


Vert. 


D.R.     Ec.gl.       Sp.c. 


Nch. 


Ve'. 


Ve". 


card. 


N.io. 


bro. 


Lu. 


Piece. 


EIG?  208. — PIG,  17.0  MM.     TRANSVERSE  SERIES  51,  SECTION  464. 

Ao,  Aorta,  bro,  Entodermal  bronchus,  card,  Posterior  cardinal  vein,  cin,  Neurone  layer  (cinerea)  of  spinal 
cord.  Cost,  Anlage  of  ribs.  D.R,  Dorsal  root.  Ec.gl,  Ectoglia.  C,  Ganglion.  Li,  Liver.  Lu,  Lung. 
muse,  Dorsal  musculature.  N.io,  Vagus  nerve.  Nch,  Notochord.  (E,  (Esophagus.  Pl.cce,  Pleural 
ccelom.  R.D,  Ramus  dorsalis.  R.V,  Ramus  ventralis.  R.sy,  Ramus  sympathicus.  Sp.c,  Spinal  cord. 
Sym,  Sympathetic  ganglion.  Ve',  Ve",  Branches  of  the  subclavian  vein.  Vert,  Vertebra.  X  22  diams. 

oped  matrix.  Each  cell  occupies  a  separate  space  or  capsule  in  the  matrix.  The 
protoplasm  of  the  cell,  having  changed  to  a  transparent  substance  and  being  un- 
stained, seems  to  have  disappeared,  but  the  nucleus  remains  distinct,  for  it  stains 
readily,  has  a  sharp  outline,  and  contains  a  number  of  dark  granules,  one  or  two 
of  which  are  conspicuous  by  their  greater  size  and  irregular  shape.  The  nucleus 
itself,  in  most  of  the  cells,  is  somewhat  irregular  in  outline,  as  if  distorted  by 

shrinkage.     Toward   the   center  of  the  anlage   the   cytomorphosis  is   most  advanced. 

. 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  J7  MM. 

more,  regularly  shaped  nuclei.  In  the  center  of  the  vertebra  lies  the  round  noto- 
chord,  Nch,  the  sheath  of  which  has  increased  considerably  in  thickness,  and, 
being  unstained,  appears  as  a  clear  space  between  the  cells  of  the  notochord 
and  those  of  the  enclosing  vertebra.  The  nuclei  in  the  notochord  are  numerous 
and  somewhat  crowded  together.  (2)  The  costal  processes,  Cost,  of  the  vertebra, 
which  are  rod-like  and  extend  quite  far  down  into  the  somatopleure.  The  histo- 
genetic  changes  in  these  processes  are  similar  to  those  in  the  vertebra,  but  less 
advanced.  They  have  progressed  somewhat  more  in  the  proximal  than  in  the 
distal  portion  of  the  rib.  (3)  Around  the  central  nervous  system  the  pia  mater 
has  become  more  distinct,  and  the  arachnoid  membrane  is  indicated  by  the  wide 
separation  of  its  cells  and  the  length  of  the  processes  connecting  them.  Its  dif- 
ferentiation is  most  easily  recognized  at  the  sides  of  the  spinal  cord.  The  outer 
limit  of  the  arachnoid  is  shown  by  a  slight  condensation  of  the  mesenchyma  which 
marks  the  first  step  in  the  differentiation  of  the  dura  mater,  the  anlage  of  which 
is  further  defined  by  the  elongated  form  of  the  mesenchymal  cells,  by  which  they 
differ  from  the  mesenchymal  cells  on  both  sides.  (4)  There  is  a  distinct  layer  of 
condensed  mesenchyma  around  the  aorta,  Ao.  The  layer  thus  formed  consists  of 
elongated  cells,  and  perhaps  corresponds  only  to  the  muscular  coat  of  the  vessel. 
(5)  About  the  oesophagus,  (E,  the  mesenchyma  forms  two  distinct  layers.  The 
inner,  next  to  the  epithelium,  is  of  looser  texture,  and  is  the  anlage  of  both  the 
mucous  and  submucous  layers  of  the  adult.  The  outer  layer  is  denser  and  con- 
sists chiefly  of  young  smooth  muscle-cells,  which  are  merely  modified  mesen- 
chymal cells,  characterized  by  the  greater  development  of  their  protoplasm  and 
by  their  elongated  form.  Traces  of  the  differentiation  of  the  outer  layer  into 
the  inner  circular  muscular  coat  and  the  outer  longitudinal  coat  of  the  adult  are 
clear  iru  the  section. 

spinal  cord,  Sp.c,  has  changed  its  outline  as  seen  in  section,  being 
brc^olest  in  the  ventral  zones,  which  have  also  begun  to  expand  ventralward 
so  Ahat  the  outline  of  the  cord  shows  on  its  ventral  side  a  concavity,  the  first 
idication  of  the  ventral  fissure.  The  three  layers  of  the  spinal  cord  are 
\Tery  distinct.  The  change  in  form,  however,  it  can  be  clearly  seen,  is  due  chiefly 
to  the  growth  of  the  gray  layer,  tin,  especially  in  the  ventral  zone.  The  gray 
layer  in  the  dorsal  zone  is  still  very  slightly  developed.  From  the  dorsal  zone 
descends  on  either  side  the  dorsal  nerve-root,  D.R,  which  presently  joins 
the  ganglion,  G.  The  ganglion  now  occupies  a  much  lower  position  than  in  the 
earlier  stages  (compare  Fig.  198,  G).  From  the  ventral  zone  springs  the  ventral 
root  which  unites  with  the  dorsal  at  the  lower  tip  of  the  ganglion.  From  the 
nerve-trunk  thus  formed  there  is  given  off  almost  immediately  the  dorsal  branch, 
R.D,  which  soon  ramifies  in  the  midst  of  a  dark  mass  of  tissue,  the  anlage  of  the 
dorsal  musculature,  muse.  The  main  nerve-trunk  descends  ventralward  and  sends 
off  at  the  level  of  the  vertebra  a  sympathetic  branch,  R.sy,  which  runs  obliquely 
downward  and  inward  toward  the  aorta,  and  there  terminates  in  the  anlage  of  the 


306  STUDY  OF  PIG  EMBRYOS. 

sympathetic  chain,  Sym,  which  consists  partly  of  nerve-fibers,  partly  of  ganglion 
cells  which  have  migrated  along  the  nerve  and  taken  up  their  position  at  its  end. 
These  cells  are  easily  recognized  by  their  very  dark  staining.  Their  nuclei  are  a 
little  lighter  than  those  of  the  neighboring  mesenchymal  cells,  but  the  cells,  owing 
to  -their  deep  coloration,  are  conspicuous  even  when  the  section  is  examined  only 
with  the  low  power.  The  sympathetic  anlage  comes  in  close  contact  with  a  por- 
tion of  the  cardinal  vein,  card,  near  the  aorta.  The  main  nerve-trunk,  R.V,  con- 
tinues obliquely  downward  and  presently  forks  into  an  upper  and  a  lower  branch. 
The  cardinal  veins,  card,  lie  on  either  side  of  the  aorta,  but  they  are  almost 
completely  obliterated  by  the  ingrowth  of  the  Wolffian  tubules,  which  subdivide 
the  vein  into  numerous  smaller  channels  or  sinusoids.  The  section  also  shows 
two  branches,  Ve' ',  and  Ve",  of  the  subclavian  vein.  The  identity  of  these  branches 
has  not  yet  been  determined.  Beneath  the  aorta,  Ao,  follows  the  oesophagus,  (E, 
the  lumen  of  which  is  much  smaller  than  that  of  the  aorta.  Its  epithelium  has 
the  general  characteristics  of  the  epithelial  entoderm  at  this  stage,  being  a  rather 
thick  cylinder  epithelium.  As  above  mentioned,  the  differentiation  of  the  mucous 
and  muscular  layers  of  the  oesophagus  shows  clearly.  Below  the  oesophagus  lie 
the  two  large  vagus  nerves,  N.io,  and  then  follow  the  sections  of  the  two  lungs, 
Lu.  Each  lung  is  a  lobe  of  tissue  connected  with  its  fellow  across  the  median 
line  of  the  embryo  and  projecting  laterally  far  into  the  pleural  cavity,  Pl.cce. 
The  lung  consists  chiefly  of  a  large  accumulation  of  dense  mesenchyma  in  which 
the  epithelial  bronchi,  bro,  ramify.  Every  bronchus  has  a  central  lumen  and  its 
walls  are  formed  by  a  moderately  thick  layer  of  cylinder  entodermal  cells.  The 
surface  of  each  lung  is  covered  by  mesothelium,  which  is  shown  as  a  distinct  line 
in  the  engraving.  The  mesothelium  can  be  followed  to  the  root  of  the  lung, 
where  it  is  reflected  on  to  the  outer  wall  of  the  pleural  chamber.  The  pleural 
cavity,  Pl.cce,  is  thus  everywhere  bounded  by  mesothelium  which  persists  through- 
out life,  being  known  in  the  adult  as  the  pleural  epithelium. 

Section  through  the  Wolffian  Body  and  Genital  Gland  (Fig.  209). — The  gen- 
eral characteristics  of  the  ectoderm,  mesenchyma,  and  nervous  system  are  nearly 
the  same  as  in  the  section  last  described.  On  one  side  the  section  shows  a  thick- 
ening of  the  ectoderm,  the  anlage  of  a  mammary  gland,  mam  (compare  page  320). 
The  branches  of  the  nerves  are  not  so  well  shown  in  this  section  as  in  the  previous 
one.  The  level  of  our  section  corresponds  to  the  lower  end  of  the  vena  cava 
inferior,  which  is  marked  at  this  stage  by  the  two  large  mesonephric  veins,  V.msn, 
which  come  from  the  Wolffian  bodies  and  by  their  union  constitute  the  lower  end 
of  the  vena  cava.  The  mesonephric  veins  are,  strictly  speaking,  portions  thereof. 
The  Wolffian  bodies  are  the  most  conspicuous  structures  shown  in  the  section. 
They  consist  chiefly  of  a  great  number  of  tubules,  W.t,  very  much  crowded  to- 
gether. On  the  median  side  of  the  organ  appear  the  large  glomeruli,  Glo,  and-  on 
their  ventral  side  we  have  the  section  of  the  longitudinal  Wolffian  duct,  W.D. 
The  tubules  of  the  Wolffian  body  are  formed  by  a  more  or  less  nearly  cuboidal 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  17  MM. 


307 


R.sy. 


Ao. 


art. 


msth. 


epithelium,  the  nuclei  of  which  are  decidedly  larger  than  those  of  the  mesenchymal 
cells.  The  nuclei  themselves  stain  deeply,  have  well-marked  outlines,  and  very 
distinct  granules  in  their  interior.  The  protoplasm  of  the  cells  also  stains  some- 
what with  cochineal,  carmine,  hematoxylin,  etc.  There  is  very  little  mesenchyma 
Nch.  ec.gl.  Sp.c.  G.  N.  R.V . 


R.V 


Sym. 


Cce. 


V.msn. 


Gen. 


Glo. 


W.t. 


Som. 


W.D. 


mst. 


In.  Li. 

FIG.  209.— PIG,  17.0  MM.     TRANSVERSE  SERIES  51,  SECTION  651. 

Ao,  Dorsal  aorta.  art,  Glomerular  artery.  Cce,  Ccelom.  ec.gl,  Ectoglia.  G,  Ganglion.  Gen,  Genital 
gland.  Glo,  Glomerulus  of  Wolffian  body.  In,  Intestine.  Li,  Liver,  mam,  Mammary  anlage1.  mst, 
Mesentery,  msth,  Mesothelium.  N,  Ventral  nerve.  Nch,  Notochord.  R.sy,  Ramus  sympathicus  of 
nerve.  R.V',  R.V",  Branches  of  the  ventral  ramus  of  the  spinal  nerve.  Som,  Somatopleure.  Sp.c, 
Spinal  cord.  Sym,  Sympathetic  ganglion.  V.msn,  Vena  mesonephrica.  W.D,  Wolffian  duct.  W.t, 
\Yolffian  tubule.  X  22  diams. 

in  the  organ,  but  each  tubule  is  closely  invested  by  vascular  endothelium;  hence  the 
tubules  are  separated  from  one  another  only  by  blood  spaces,  which,  morphologi- 
cally speaking,  are  portions  of  the  cavity  of  the  cardinal  vein.  These  blood  spaces 
are  highly  characteristic  and  are  typical  sinusoids.  The  intertubular  circulation 
of  the  Wolffian  body  is,  so  far  as  known,  always  sinusoidal.  The,,  aorta,  Ao,  is 
seen  in  the  figure  to  give  off  a  small  branch, '  art,  which  runs  toward  the  Wolffian 
body.  There  are  numerous  such  branches,  each  one  of  which  may  be  traced  to  a 


308  STUDY  OF  PIG  EMBRYOS. 

glomerulus  of  the  mesonephros.  Each  glomerulus  has  a  capillary  circulation, 
and  the  blood  on  leaving  the  glomerulus  is  supposed  to  be  emptied  into  the  venous 
sinusoids.  More  exact  investigation  of  .  this  point  is  needed.  The  mesonephros 
is  covered  by  a  layer  of  mesothelium,  msth,  underneath  which  is  a  thin  layer  of 
mesenchyma.  The  two  together  constitute  the  anlage  of  the  peritoneal  covering 
of  the  organ.  To  the  median  side  of  the  Wolffian  body  is  appended  the  large 
anlage  of  the  genital  gland,  Gen,  which  has  a  constricted  connection  with  the 
Wolffian  body.  Each  gland  is  covered  by  mesothelium  and  extends  until  it  comes 
in  contact  with  the  mesentery,  mst.  The  gland  contains  two  kinds  of  tissue,  one, 
the  anlage  of  the  medullary,  the  other  of  the  cortical  portion  of  the  gland.  The 
medullary  tissue  resembles  the  neighboring  mesenchyma  and  occupies  only  a  small 
territory  about  the  stalk  of  the  organ.  The  cortical  tissue  contains  cells  with  much 
larger  nuclei  and  clearly  developed  protoplasmic  bodies.  It  occupies  by  far  the 
larger  part  of  the  gland.  Comparison  with  figure  198  will  show  that  the  genital 
anlage  at  this  stage  occupies  -the  same  topographical  relation  to  the  Wolffian  body 
as  at  earlier  stages.  It  differs  now  from  the  earlier  condition  chiefly  by  its  growth 
in  size  and  by  its  advancement  in  histological  differentiation.  Below  the  genital 
gland  the  intestinal  canal  is  cut  several  times.  One  portion  of  the  intestine  is 
seen  in  the  section  to  be  connected  by  means  of  the  mesentery,  mst,  with  the 
median  dorsal  tissues  of  the  embryo.  The  intestine  is  formed  by  a  small  tube 
of  entoderm  with  a  small  cavity.  The  entoderm  is  a  rather  thick  cylinder  epithe- 
lium. The  greater  part  in  bulk  of  the  walls  of  the  intestine  is  constituted  by 
mesenchyma.  The  external  surface  is  covered  by  a  thin  mesothelial  layer.  The 
mesenchyma  is  beginning  to  show  the  differentiation  of  the  external  muscular 
from  the  internal  mucous  coat.  There  is  at  this  stage  no  trace  whatever  of  the 
development  of  any  folds  or  glands  on  the  inside  of  the  intestinal  canal. 

Section  through  the  Kidney  (Fig.  210). — This  section  being  much  nearer  the 
caudal  end  of  the  embryo,  we  find,  as  throughout  all  the  early  stages,  that  the 
differentiation  of  the  tissues  is  less  advanced  than  nearer  the  head.  We  have 
accordingly,  so  to  speak,  an  earlier  stage  in  the  development  of  the  spinal  cord, 
Sp.c,  -of  the  nerves,  and  of  the  vertebra.  In  the  median  line  is  the  large  aorta, 
Ao,  about  which  the  mesenchyma  is  only  slightly  condensed.  Near  the  aorta  are 
the  conspicuous  anlages  of  the  sympathetic  system,  Sym,  which  appear  at  this  level 
in  a  very  characteristic  hook-shaped  pattern.  At  the  dorsal  end  of  the  hook  the 
nerve-fibers  are  much  more  numerous  than  in  the  ventral  portion  of  the  anlage. 
The  sympathetic  cells  themselves  are  extremely  conspicuous,  owing  to^  the  depth  of 
their  stain.  On  either  side  is  situated  the  anlage  of  the  permanent  kidney,  Ki. 
Each  anlage  consists  of  an  irregularly  branching  space  bounded  by  a  thick  layer 
of  epithelium,  which  has  somewhat  the  appearance  of  the  intestinal  entoderm  at 
this  stage.  If  the  series  of  sections  be  followed  through  farther  toward  the  tail 
of  the  embryo,  the  epithelial  space  will  be  seen  to  contract  to  a  relatively  small 
tube,  the  ureter,  which  opens  into  the  Wolffian  duct  of  the  same  side.  The  ex- 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  17  MM. 


309 


panded  portion  of  the  cavity  shown  in  our  figure  corresponds  in  part  to  the  pelvis 
of  the  adult  organ.  Its  irregular  shape  is  due  to  the  fact  that  it  is  forming  a 
series  of  outgrowths,  which  are  to  give  rise  to  the  collecting  tubules.  Around 
the  ends  of  the  branches  of  the  renal  pelvis  is  a  darker  tissue,  in  which  the  cells 
are  very  much  crowded.  It  is  the  material  out  of  which  the  glomeruli  and  con- 
voluted tubules  of  the  kidney  are  to  be  differentiated.  By  a  secondary  process 
these  tubules  become  united  with  the  branches  from  the  renal  pelvis,  the  branches 
forming  the  collecting  tubules  only  of  the  adult  organ  (compare  page  no).  The 


Nch 


W.D 


FIG.  210. — PIG,  17.0  MM.     TRANSVERSE  SERIES  51,  SECTION  759. 

All]  Allantois.  Ao,  Aorta.  A.um,  Umbilical  artery,  card,  Branch  of  cardinal  vein.  Cce,  Coelom.  G,  Gan- 
glion. Ki,  Kidney.  N',NV,  Nerves.  Nch,  Notochord.  P.L,  Posterior  limbs.  Reel,  Large  intestine.  Sp.c, 
Spinal  cord.  Svm,  Sympathetic  ganglion.  W.b,  Wolffian  body.  W.D,  Wolffian  duct.  X  17  diams. 

origin  of  the  renal  anlage  may  easily  be  followed  in  earlier  stages.  It  is  found  that 
from  the  pelvic  end  of  each  Wolffian  duct  there  develops  a  dorsal  outgrowth,  which 
is  lined  by  epithelium.  This  outgrowth  elongates  in  a  headward  direction.  Its 
end  expands;  the  narrow  portion  is  the  ureter,  the  expanded  portion  the  anlage 
of  the  pelvis.  The  pelvis  becomes  irregular  in  shape  and  forms  outgrowths. 
Around  it  appears  the  condensed  tissue  just  referred  to.  On  the  ventral  and 
lateral  sides  of  the  kidneys  in  our  section  appear  the  ends  of  the  Wolffian  bodies, 
W.b.  From  the  ventral  and  inner  edge  of  each  Wolffian  body  is  a  projecting  lobe 
of  tissue  in  which  the  Wolffian  duct,  W.D,  is  lodged.  The  walls  of  the  Wolffian 
duct  are  a  rather  thin,  cuboidal  epithelium,  surrounded  by  mesenchyma  in  which 
there  is  no  very  clear  evidence  of  specialization.  Between  the  Wolffian  bodies 


310 


STUDY  OF  PIG  EMBRYOS. 


is   suspended   the   large   intestine.     It   has   a    small   canal   formed   by   entoderm    and 
very   thick   mesodermic   walls. 

Attached  to  the  ventral  side  of  the  body- wall  of  the  embryo  is  the  allantois, 
All,  the  cavity  of  which  is  quite  large,  somewhat  irregular  in  shape,  and  lined 
by  a  cuboidal  epithelium,  a  portion  of  the  entoderm.  By  following  through  the 
sections  it  can  be  seen  that  the  allantois  and  large  intestine  join  at  the  cloaca. 
The  entodermal  allantois  is  surrounded  by  mesenchyma,  which  is  very  much 
looser  in  texture  than  that  of  the  intestine  proper.  On  either  side  of  the  allantois 
is  a  projecting  lobe  of  tissue  in  which  the  umbilical  artery,  A.um,  is  lodged.  The 


A.m. 


V.vi. 


Cce. 


Art. 


U.V.S.  All. 

FIG.  211. — PIG,  17.0  MM.    FRONTAL  SERIES  39,  SECTION  64. 

A II,  Allantois.     Art,  Umbilical  artery.     A  .vi,  Vitelline  artery.     Cce,  Ccelom.     EC,  Ectoderm.     //,  Ileum.     mes, 
Mesenchyma.     U.V.S,  Left  umbilical  vein.     V.vi,  Vitelline  vein.       X  35  diams. 

two  arteries  pass  upward  to  the  umbilicus,  then  outward  to  the  placenta.  Down- 
ward they  continue  to  the  level  of  the  cloaca,  there  pass  to  the  dorsal  side  of 
the  embryo,  and  unite  with  the  end  of  the  median  dorsal  aorta. 

Frontal  Section  of  the  Umbilical  Cord  (Fig.  211). — We  get  in  frontaj  series 
of  the  embryo  sections  of  the  umbilical  cord  which  are  more  or  less  nearly  trans- 
verse. The  major  part  of  the  area  of  such  sections  is  occupied  by  mesenchyma, 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  20  MM.  311 

Mes.  On  the  ventral  side  of  the  cord  is  the  cavity  of  the  allantois,  All,  lined  by 
a  thin  layer  of  entoderm,  and  with  no  marked  condensation  of  the  mesenchyma 
around  it.  A  little  lower  is  the  large  umbilical  vein,  U.V.S,  which  appears  as  a 
prolongation  of  the  left  umbilical  of  the  body  proper,  but  the  part  within  the  cord 
is  probably  the  product,  of  the  fusion  of  the  two  original  veins.  A  little  higher  are 
the  two  umbilical  arteries,  Art,  which  lie  symmetrically  as  regards  the  allantois. 
All  three  vessels  are  strengthened  by  walls  of  condensed  mesenchyma,  which  is 
much  more  prominent  around  the  arteries  than  around  the  vein.  The  center  of 
the  cord  is  occupied  by  a  large  irregular  space,  Cce,  a  prolongation  of  the  body 
cavity.  In  this  umbilical  ccelom  are  lodged  the  loop  of  the  intestine  and  the  cord 
containing  the  vitelline  vein.  The  intestine  is  cut  twice,  the  section  on  the  left 
passing  through  the  ileum,  II,  and  on  the  right  through  the  jejunum,  which  is 
much  larger  than  the  ileum,  having  both  a  larger  entodermal  portion  and  a  thicker 
mesodermal  part.  The  two  segments  of  the  intestine  are  joined  together,  and  in 
the  part  between  them  are  two  blood-vessels,  one,  the  inferior,  is  the  vitelline 
artery,  A.vi,  which  extends  beyond  the  intestinal  loop,  to  ramify  upon  the  yolk- 
sac;  the  other  vessel  is  the  superior  mesenteric  vein,  which  does  not  extend  be- 
yond the  intestine.  The  mesenchyma  of  the  intestines  and  of  the  bit  of  mesentery 
between  them  consists  of  very  crowded  cells,  so  that  the  tissue  appears  darkly 
stained.  Above  the  loop  lies  the  cord  in  which  is  situated  the  vitelline  vein,  V.vi. 
The  vein  is  centrally  placed;  the  cord  forms  a  thick  wall  of  very  loose  mesenchyma 
.  covered  by  a  thin  mesothelial  layer.  The  cord  is  a  very  characteristic  embryonic 
structure;  it  arises  from  the  mesentery  of  the  duodenum  and  extends  through  the 
umbilical  opening  to  the  yolk-sac.  The  vein  which  it  contains  is  thought  to 
arise  by  the  fusion  of  the  two  original  vitelline  or  omphalo-mesaraic  veins.  Its 
union  with  the  superior  mesenteric  vein  to  form  the  portal  vein  is  shown  in  figure 
100.  All  the  surfaces  of  the  ccelom  are  covered  by  a  distinct  mesothelium.  The 
main  tissue  of  the  umbilical  cord  is  a  typical  loose  mesenchyma,  Mes.  The  ecto- 
derm, EC,  is  the  direct  prolongation  of  the  embryonic  epidermis,  and  consists  for 
the  most  part  of  a  single  layer  of  cells,  although  the  formation  of  a  second  outer 
layer  seems  to  be  beginning. 

Pig  Embryo  of  20  mm.     Study  of  Sections. 

Nine  sections  of  this  stage  are  figured.  In  the  practical  laboratory  work  em- 
bryos a  little  larger  or  smaller  may  serve  equally  well  to  illustrate  the  develop- 
mental conditions  of  this  stage. 

Transverse  Section  through  the  Snout  (Fig.  212). — The  parts  shown  are  the  same 
as  in  figure  219,  to  the  description  of  which  reference  is  made.  The  present  figure 
212  is  added  to  illustrate  the  development  of  the  palate  shelf,  Pal.  The  palate 
shelf  is  a  large  protuberance  on  the  inner  side  of  the  maxillary  process.  Its  inner 
edge  abuts  against  the  tongue,  Ton,  its  upper  edge  underlies  the  maxillo-turbinal 
fold,  max.tb,  and  its  lower  edge  forms  part  of  the  roof  of  the  oral  cavity.  Or. 


312 


STUDY  OF  PIG  EMBRYOS. 


At  this  stage  it  consists  of  a  large  mass  of  imdifferentiated  mesenchyma,  covered 
by  a  layer  of  epithelium.  The  two  palate  shelves  continue  to  grow  toward  one 
another  until  they  meet  in  the  median  line  below  the  nasal  septum,  Sept.  As  they 
approach  one  another  the  tongue  descends.  Ultimately  the  two  palate  shelves 
unite  with  one  another  and  with  the  overlying  nasal  septum.  The  epithelium  of 
the  two  shelves  concresces  and  forms  for  a  time  a  partition,  which  marks  the  point 
of  union  of  the  two  shelves,  both  with  one  another  and  with  the  nasal  septum. 
This  partition  persists  for  a  short  time  only,  for  it  soon  disappears  by  resorption. 


nas.tb. 


Sept. 


max.tb. 


Pal. 


Ton. 


Or. 


Mk. 


FIG.  212. — PIG,  20  MM.     TRANSVERSE  SERIES  59,  SECTION  522. 

Jk.o,  Jakobson's  organ,  lat,  Lateral  ethmoid  cartilage,  max.tb,  Maxillo-turbinal  fold.  Mk,  Meckel's  cartilage. 
nas.tb,  Naso-turbinal  fold.  Or,  Oral  cavity.  Pal,  Palate  shelf.  Sept,  Cartilage  of  nasal  septum.  Ton, 
Tongue.  X  22  diams. 

The  union  of  the  palate  shelves  separates  definitely  the  nasal  and  oral  cavities 
from  one  another.  Their  union  is  gradual,  beginning  in  front  and  gradually  ex- 
tending backward.  It  is  a  not  infrequent  anomaly  that  the  palate  shelves  fail  to 
unite  perfectly.  When  this  occurs,  there  results  the  condition  known  as  cleft  palate. 
Transverse  Section  through  the  Lower  Part  of  the  Neck  (Fig.  213). — The  spinal 
cord,  Sp.c,  shows  a  very  great  enlargement  of  the  ventral  zones,  which  now  pro- 
ject downward  so  as  to  enclose  between  them  a  distinct  groove  in  the  median  ven- 
tral line,  which  can  be  identified  as  the  commencing  anterior  fissure  of  the  cord. 
In  this  groove  runs  a  small,  longitudinal  blood-vessel,  the  arteria  sulci,  which  from 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  20  MM. 


313 


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314  STUDY  OF  PIG  EMBRYOS. 

time  to  time  gives  off  small  branches,  which  enter  the  substance  of  the  spinal  cord. 
In  the  ventral  zone  the  ependymal  layer  has  become  quite  thin  and  the  middle  or 
gray  layer  has  acquired  great  thickness,  chiefly  owin^  to  the  growth  of  the  neuro- 
blasts,  many  of  which,  especially  toward  the  outside  of  the  cord,  can  now  be 
readily  identified  as  young  nerve-cells.  The  ectoglia  or  outer  neuroglia  layer  has 
increased  in  thickness.  Many  of  the  processes  of  the  neuroglia  cells  can  be 
readily  distinguished,  running,  for  the  most  part,  more  or  less  nearly  perpendicular 
to  the  surface  of  the  cord.  Between  the  neuroglia  fibers  are  numerous  fine  dots 
which  are  the  cut  ends  of  the  nerve-fibers  running  longitudinally.  Although  about 
these  nerve-fibers  there  are  as  yet  no  medullary  sheaths  developed,  it  is,  never- 
theless, proper  to  speak  now  of  the  ectoglia  as  the  external  white  matter  of  the 
cord.  Immediately  beneath  the  entrance  of  the  dorsal  root  the  external  outline 
of  the  cord  shows  a  concavity  which  disappears  in  later  stages.  The  dorsal 
zones  are  very  much  smaller  than  the  ventral.  The  differentiation  of  their  three 
primary  layers  is  being  completed  by  the  development  of  a  distinct  middle  layer. 
The  ectoglia  of  the  dorsal  zone  resembles  that  of  the  ventral  zone  in  structure 
and  thickness.  The  spinal  ganglia,  G,  have  descended  from  their  original  position, 
so  that  they  now  lie  on  a  level  with  the  lower  edge  of  the  spinal  cord,  and  the 
nerve-root,  by  which  each  ganglion  is  connected  with  the  dorsal  zone  of  the  cord, 
has  correspondingly  elongated.  The  lower  edges  of  the  ganglia  come  in  contact 
with  the  lateral  processes  of  the  vertebra.  Between  the  spinal  cord  and  the 
vertebra  is  an  area  of  loose  mesenchyma  which  may  be  regarded  as  a  portion  of 
the  arachnoid  membrane.  Close  to  the  upper  surface  of  the  vertebra,  bounded 
dorsally  by  the  tissue  just  mentioned,  are  two  symmetrically  placed  blood-vessels. 
The  intervertebral  ligament,  Iv.D,  is  only  partially  cut.  Above  it  appears  the 
lighter  tissue  of  the  next  following  vertebra,  which  is  shown  better  several  sections 
lower  down.  The  vertebra  is  distinctly  cartilaginous,  though  not  yet  fully  differ- 
entiated, and  is  surrounded  by  a  distinct  fibrous  layer,  the  perichondrium.  In 
the  median  line  below  the  vertebra  lie 'the  oesophagus,  ffi,  and  trachea,  Tra,  both 
tubes  lined  by  entoderm.  The  cavity  of  the  oesophagus  is  somewhat  crescent- 
shaped,  that  of  the  trachea  triangular.  About  the  oesophagus  the  mesoderm  forms 
two  layers,  an  inner  lighter  layer  and  an  outer  muscular  layer,  the  cells  of  which 
are  already  elongated.  The  mesenchyma  about  the  trachea  is  more  condensed, 
especially  on  the  sides  and  below,  and  the  condensed  tissue  is  in  close  contact 
with  the  epithelium.  On  the  dorsal  side  of  the  trachea  close  to  the  entoderm  is 
a  thin  layer  of  transversely  elongated  cells.  The  sympathetic  nervous  system,  Sym, 
appears  symmetrically  placed  near  the  trachea  and  oesophagus.  In  section  the 
sympathetic  is  round  and  contains  numerous  nerve-fibers  and  characteristic  young 
sympathetic  nerve-cells,  by  which  it  is  readily  recognized.  Close  to  the  ventral 
side  of  the  sympathetic  is  the  section  of  the  large  jugular  vein,  V.jug,  a  branch  of 
which,  V.br,  lies  laterad  from  the  main  vessel.  This  branch  receives  blood-ves- 
sels from  the  facial  region.  Between  the  main  jugular  and  its  branch  are  some 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  20  MM.  315 

lymphatic  spaces,  somewhat  irregular  in  form,  and  lined  by  a  thin  endothelium  so 
that  they  present  a  close  resemblance  to  veins  in  their  structure;  if  followed  up 
toward  the  head  the  lymphatics  are  found  to  unite  with  the  large  jugular  lymph- 
sac  (Fig.  60  ,  s.l.f).  Close  to  the  medial  wall  of  the  jugular  vein  is  situated  the 
large  trunk  of  the  vagus  nerve,  N.io.  At  a  little  lower  level  than  the  vagus 
nerves  and  in  the  median  line  lies  the  anlage  of  the  thyroid  gland,  which,  owing 
to  its  darker  staining,  is  somewhat  conspicuous.  The  cells  of  the  thyroid  form 
an  irregularly  shaped  branching  mass.  The  spaces  between  the  branches  are  chiefly 
occupied  by  small  endothelial  blood-vessels.  The  arrangement  of  these  cavities 
and  the  relation  of  their  endothelium  to  the  cells  of  the  organ  recall  the  blood 
sinusoids  of  the  liver  and  of  the  suprarenal  capsule.  The  thyroid  cells  are  com- 
pactly arranged  without  distinct  cell-boundaries,  but  with  protoplasm  which  stains 
somewhat  and  with  nuclei  of  rounded  form,  distinct  outline,  and  granular  appear- 
ance, the  granules  being  decidedly  more  conspicuous  than  the  granules  in  the  nu- 
clei of  the  neighboring  mesenchymal  cells.  Just  ventral  to  each  jugular  vein  is 
a  small  darker  body,  consisting  of  closely  compacted  cells,  resembling  in  appear- 
ance those  of  the  thyroid.  The  body  has  a  very  distinct  external  outline  and 
is  actively  growing,  for  several  of  its  nuclei  are  in  mitosis.  The  bodies  in  question 
are  the  parathyroid  glands.  The  rest  of  the  section  is  mainly  occupied  by  mesen- 
chyma and  numerous  darker  masses,  muse,  the  anlages  of  the  various  muscles  of 
the  neck  and  throat.  On  each  side  is  shown  a  small  piece  of  the  cartilaginous 
scapula,  Scap.  At  the  lower  corner  of  the  section  is  an  indication  of  the  anterior 
limb,  A.L,  and  of  its  vein,  Ve". 

Section  through  the  Lungs  (Fig.  214). — The  spinal  cord  shows  very  clearly  in 
the  differentiation  of  the  three  primary  layers  of  the  medullary  wall.  Its  structure 
is  similar  to  that  shown  in  figure  208,  and  need  not  be  again  described.  The 
vertebra,  Vert,  is  now  distinctly  young  cartilage.  On  its  ventral  side  its  boundary 
is  quite  distinct,  the  formation  of  the  perichondrium  having  there  begun.  Laterally 
it  merges  into  a  dense  mesenchyma,  by  which  it  is  united  without  demarcation  with 
the  rib,  cost',  and  indirectly  with  the  vertebral  arch,  V.ar,  both  of  which  are 
cartilaginous.  The  cells  of  the  vertebral  cartilage  occupy  rounded  cavities,  each 
of  which  is  marked  by  a  distinct  capsule.  The  matrix  between  the  capsules  is 
homogeneous,  stains  slightly,  and  has  acquired  a  "greater  density  than  in  earlier 
stages.  The  cells  themselves  exhibit  traces  of  their  protoplasmic  bodies  and  have 
deeply  stained  nuclei  which  are  quite  irregular  in  shape  and  very  granular.  Im- 
mediately around  the  notochord  the  spaces  occupied  by  the  cells  are  the  largest, 
the  capsules  most  distinct,  and  the  nuclei  most  altered.  Proceeding  toward  the 
periphery  of  the  cartilage,  the  cells  appear  in  successively  earlier  and  earlier  stages, 
until  at  the  very  periphery  we  have  normal  nuclei  and  a  transition  to  mesenchyma. 
The  notochord  has  contracted,  leaving  a  space  between  the  notochordal  cells  and  the 
vertebral  cartilage.  Immediately  below  the  vertebra  are  the  conspicuous  anlages 
of  the  sympathetic  system,  Sym.  They  overlie  the  sections  of  the  posterior  cardinal 


310 


STUDY  OF  PIG  EMBRYOS. 


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STUDY  OF  SECTIONS  OF  EMBRYO  OF  20  MM.  317 

veins,  card.  These  are  now  quite  small  vessels,  the  vena  cava  inferior  having 
become  the  main  channel  for  the  return  of  the  blood  from  the  abdominal  region  to 
the  heart.  The  two  cardinal  veins  are  not  quite  symmetrically  placed,  that  on  the 
left  side  lying  a  little  lower  than  that  on  the  right.  Between  them  is  situated 
the  median  aorta,  Ao,  with  a  relatively  thick  and  well-developed  muscular  coat, 
the  deeper  staining  of  which  makes  it  conspicuous  even  with  low  powers.  The 
(esophagus,  (E,  and  trachea,  Tra,  are  not  in  the  median  line,  but  are  both  dis- 
placed toward  the  right  of  the  embryo..  As  compared  with  earlier  stages,  both 
structures  show  an  advance,  first,  by  the  growth  of  the  entoderm,  and,  second,  by 
the  differentiation  of  the  surrounding  mesenchyma.  In  both  oesophagus  and  trachea 
the  entoderm  is  a  ring  of  cylinder  epithelium,  the  tracheal  ring  being  much  larger 
than  the  oesophageal.  The  mesenchyma  about  the  oesophagus  forms  two  distinct 
layers,  an  inner  looser  layer  and  an  outer  denser  muscular  layer.  Around  the 
trachea  the.  mesoderm  is  much  condensed.  On  the  dorsal  side  of  the  trachea  the 
cells  form  next  to  the  epithelium  a  special  layer  characterized  by  the  elongated 
form  of  the  cells.  Between  the  oesophagus  and  trachea  are  situated  the  vagus  nerves, 
that  of  the  right  side,  N.io,  occupying  a  higher  position  than  that  on  the  left, 
so  that  the  nerves  are  not  symmetrically  placed.  The  cardinal  veins,  the  aorta, 
the  oesophagus,  the  vagus  nerve,  and  the  trachea  are  all  imbedded  in  mesenchyma, 
which,  together-  with  these  structures,  forms  the  so-called  mediastinum  by  which  the 
right  and  left  pulmonary  cavities,  Pl.d,  Pl.s,  are  separated  from  one  another.  On 
its  ventral  side  the  mediastinum  joins  on  to  the  veins  entering  the  heart.  On  either 
side  of  the  mediastinum  at  the  level  of  the  trachea  may  be  seen  the  projecting 
lung.  That  on  the  left  side  shows  clearly  the  division  of  the  organ  into  a  dorsal 
lobe,  Lu.d,  and  a  ventral  lobe,  Lu.v.  Each  lung  consists  at  this  stage  chiefly  of 
mesenchymal  tissue  and  is  covered  by  a  layer  of  mesothelium  which  forms  the 
boundary  of  the  pleural  ccelom.  Within  the  mesenchyma  appear  several  sections 
of  the  branches  of  the  entodermal  bronchi.  Each  bronchus  is  lined  at  this  stage 
by  a  rather  thick  entodermal  layer  of  cylinder  cells.  The  union  of  the  lung  with 
the  mediastinum  constitutes  the  so-called  root  of  the  lung.  In  the  root  of  the  lung 
is  seen  the  small  pulmonary  artery,  A.pul.  The  two  arteries  join  a  little  nearer  the 
head  and  on  the  left  side  of  the  embryo  to  form  a  single  trunk,  the  main  pul- 
monary artery.  Originally  the  pulmonary  arteries  arise  symmetrically  as  branches 
from  the  fifth  aortic  arches.  They  soon  unite,  however,  throughout  the  greater 
part  of  their  extent,  forming  a  single  vessel.  The  two  arteries  shown  in  our  figure 
represent  the  two  original  symmetrical  vessels  where  they  are  about  to  enter  the 
lungs.  On  the  ventral  side  of  the  section  various  cardiac  structures  are  shown, 
but  so  cut  that  the  picture  is  not  very  instructive.  It  will  suffice  to  refer  to  the 
explanation  of  the  figure  for  the  identification  of  the  parts. 

Sections  through  the  Miillerian  Ducts  (Fig.  215). — The  female  or  Miillerian 
ducts  are  remarkable  for  their  late  development.  In  the  12  mm.  pig  the 
small  funnel-shaped  in  aginations  of  the  mesothelium,  which  represent  the  first 


318 


STUDY  OF  PIG  EMBRYOS. 


anlages  of  the  ducts,  can  just  be  recognized  on  the  lower  mesial  surface  of  the 
Wolffian  body  near  the  cephalic  end  of  the  organ.  In  the  20  mm.  pig  the  funnels 
have  lengthened  into  tubes,  which  run  a  short  distance  caudad,  close  underneath 
the  Wolffian  duct.  Figure  215  A  is  a  section  through  the  right  Mullerian  funnel, 
F.  The  mesothelium  near  the  funnel  is  considerably  thickened,  forming  the  so- 
called  tubal  band,  Ep.  The  funnel  is  lodged  in  a  small  ridge,  Rid,  which  projects 
downward  from  the  mesonephros.  The  Wolffian  duct  does  not  appear  in  the  section 
as  it  does  not  extend  so  far  head  ward.  Figure  215  B  is  a  section  farther  caudad, 
to  show  the  oval  Mullerian  duct,  M.D,  which  immediately  underlies  the  Wolffian 
duct,  W.D,  and  causes  a  small  protuberance  on  the  surface  of  the  mesonephros. 


V.s.c.        Msth 


W.t 


Rid 


W.D 


M.D 


Rid 

FIG.  215. — PIG  OF  20  MM.    TRANSVERSE  SERIES  59,  A,  SECTION  851;  B,  SECTION  887. 

Ep,  Tubal  band  of  thickened  mesothelium.  F,  Funnel-shaped  opening  of  the  Mullerian  duct.  M.D,  Mullerian 
duct.  Msth,  Mesothelium.  Rid,  Ridge  containing  the  Mullerian  funnel.  V.s.c,  Sub-cardinal  vein.  W.D, 
Wolffian  duct.  W.I,  Wolffian  tubules.  X  150  diams. 

A  short  distance  farther  on  the  duct  ends  in  a  blind  point.  It  is  destined  to 
continue  its  backward  elongation  until  it  reaches  and  joins  the  neck  of  the  allantois. 
At  just  what  stage  the  junction  occurs  in  the  pig  is  undetermined. 

Section  through  the  Posterior  Limbs  (Fig.  216). — Although  this  section  is  from 
a  transverse  series,  yet,  owing  to  the  curvature  of  the  body,  it  shows  the  spinal 
cord  cut  very  obliquely.  The  three  layers  of  the  cord,  the  ependymal,  epen,  the 
cinerea  or  neurone  layer,  Cin,  and  the  ectoglia  are  well  marked.  Something  of 
the  dorsal  root,  D.R,  and  of  the  ganglia,  G,  of  a  lumbar  nerve  are  also  shown  in 
the  section.  The  nerves  have  already  joined  together  to  form  a  very  complex 
lumbar  plexus,  sections  of  portions  of  which  appear  at  various  points.  These  are 
all  indicated  by  the  reference  letter  N  in  the  figure,  it  being  thought  not  desirable 
to  attempt  an  identification  of  each  component  of  the  plexus.  The  plexus  is  more 
or  less  symmetrically  placed  on  tho  right  and  left,  at  about  the  level  of  the  intes- 
tine, Reel.  The  limbs  are  large  projections  extending  do\w  \vard  and  containing  in 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  20  MM. 

Cin.  D.R.  epen.  arach.  Sk.  EC. 


319 


V  .arc. 


A.sul. 


Vert.  _ 


Nch. 


muse. 


Sym. 


V.il. 


cart". 


V.p. 


Pen. 


FIG.  216. — PIG,  20  MM.    TRANSVERSE  SERIES  59,  SECTION  1253. 

arach,  Arachnoid  membrane.  A.sul,  Arteria  sulci.  cart',  cart",  Cartilaginous  anlages  of  elements  of  the  skeleton 
of  the  limb.  Cin,  Neurone  layer  of  spinal  cord.  D.R,  Dorsal  root.  EC,  Ectoderm,  epen,  Ependymal  layer 
of  spinal  cord.  G,  Spinal  ganglion,  muse,  One  of  the  muscular  anlages.  N,  N,  N,  Nerves  of  the  lumbar 
plexus.  Nch,  Notochord.  Pen,  Penis.  Red,  Rectum.  Sk,  Anlage  of  the  dura  mater.  Sym,  Sympathetic 
nerve.  T,  Tail.  Ur,  Urethra.  V.drc,  Vertebral  arch.  Vert,  Vertebra.  V.il,  Iliac  vein.  V.p,  Border 
vein  of  the  limb.  X  22  diams. 


320  STUDY  OF  PIG  EMBRYOS. 

their  interior  the  cartilaginous  anlages,  cart',  cart",  of  the  skeleton  of  the  limb, 
and,  around  these,  darker  masses  of  tissue,  the  developing  muscle-fibers.  At  the 
lower  edge  of  each  limb  is  a  blood-vessel,  V.p,  the  so-called  border  or  peripheral 
vein,  which  extends  completely  around  the  edge  of  the  developing  hand  and  foot. 
When  the  digits  are  developed,  this  vein  becomes  broken  up,  and  out  of  its  divi- 
sions are  formed  the  digital  vessels.  The  section  also  passes  through  the  penis, 
Pen,  in  the  center  of  which  is  the  urethra,  Ur.  It  shows  here  as  a  narrow 
epithelial  band  without  any  cavity,  except  a  very  small  one  at  its  external 
dorsal  end.  The  band  is  lighter  in  the  center,  owing  to  the  fact  that  the  nuclei 
are  grouped  chiefly  close  to  the  two  surfaces  of  the  band.  At  the  base  of  the  limb 
is  situated  the  irregularly  shaped  section  of  the  iliac  vein,  V ' .11.  In  the  median 
line  may  be  noted  the  following  structures.  Immediately  underneath  the  nervous 
system  is  the  arteria  sulci,  A.sul.  The  vertebra,  Vert,  and  notochord,  Nch,  resem- 
ble corresponding  structures  in  the  section  described  on  page  315,  except  that  their 
cytomorphosis  is  slightly  less  advanced.  Below  the  vertebra  lie  the  paired  anlages  of 
the  sympathetic  nervous  system,  Sym,  between  which  is  the  small  median  caudal 
artery.  The  intestine,  Reel,  has  its  transverse  diameter  somewhat  increased,  so  that 
it  appears  oval  in  the  section.  Around  it  is  beginning  the  differentiation  of  the 
mucosa  and  muscularis. 

Section  through  the  Mammary  Anlage  (Fig.  217). — The  figure  represents  a  section 
through  the  somatopleure  of  the  embryo  in  the  region  of  a  mammary  gland.  The 
ectoderm,  EC,  covers  the  external  surface  of  the  somatopleure,  as  does  the  meso- 
thelium,  msth,  the  inner  surface,  the  space  between  the  two  covering  layers  being 
occupied  by  various  mesodermic  structures.  The  ectoderm  consists  of  two  or 
three  layers  of  cells,  the  external  one  of  which,  Eptr,  the  epitrichium,  is  very  thin. 
To  form  the  mammary  anlage,  Mam,  the  ectoderm  suddenly  thickens  and  projects 
somewhat  outward  and  still  more  inward  into  the  mesoderm.  The  epitrichium 
passes  continuously  over  the  thickening,  in  the  production  of  which  it  takes  no 
share.  The  inner  edge  of  the  ectoderm  is  marked  by  a  very  distinct  line  or  base- 
ment membrane,  b,  against  the  underlying  mesoderm.  The  cells  of  the  anlage 
form  two  groups,  one  a  band  next  to  the  basement  membrane,  in  which  the  cells 
present  a  somewhat  radial  arrangement,  and  the  other  a  central  group  of  cells,  many 
of  which  are  elongated  in  a  direction  somewhat  parallel  to  the  surface  of  the 
anlage,  so  that  they  form  curving  lines.  The  elongated  cells  in  later  stages  gradu- 
ally cornify  and  fall  out,  so  that  the  anlage  becomes  hollow,  but  its  excavation 
proceeds  very  slowly,  and  in  man  is  not  usually  completed  until  after  birth.  Soon 
after  the  hollowing  out  of  the  anlage  has  begun,  it  sends  out  a  series  of  buds  from 
its  inner  surface.  These  buds  become  elongated,  somewhat  twisted  cords  of  cells, 
and  offer  at  this  stage  resemblance  to  embryonic  sweat-glands.  The  outgrowths 
subsequently  branch  and  develop  central  cavities,  and  are  ultimately  transformed 
into  'the  secretory  portion  of  the  gland. 

Figure    217    also  illustrates    some    important  points  in  regard   to  the   differentia- 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  20  MM. 


321 


tion  of  the  somatopleure.  Parallel  to  the  ectoderm,  and  some  distance  from  it,  is 
a  layer,  Pan,  which  is  marked  out  by  numerous  blood-vessels.  This  is  the  pan- 
choroid  layer.  There  is  a  slight  but  unmistakable  difference  in  the  mesoderm  within 
and  without  this  layer,  for  in  the  region  between  the  panchoroid  and  the  ectoderm 
the  cells  are  somewhat  more  crowded.  They  probably  represent  the  anlage  of 


Mam. 


b. 


FIG.  217. — PIG,  20. o  MM.     TRANSVERSE  SERIES  59,  SECTION  1043. 

b,  Basement  membrane  of  epidermis,  -cost,  Costal  anlage.  Cu,  Cutis.  EC,  Ectoderm.  Eptr,  Epitrichium, 
Mam,  Mammary  anlage.  tnes,  Mesenchyma.  msth,  Mesothelium.  Pan,  Tunica  panchoroidea.  per. in. 
Peritoneal  mesoderm.  v e,  Blood-vessel.  X  250  diams. 

the  cutis,  Cu,  and  of  the  cuds  only.  Within  the  vascular  layer  the  mesodermic 
cells,  mes,  are  not  so  near  to  one  another,  and  the  processes,  by  which  they  are 
connected,  are  more  slender.  Toward  the  mesothelium  is  a  broad  band  of  denser 
tissue,  cost,  the  rudiment  of  a  rib,  the  inner  boundary  of  which  is  further  marked 
by  several  blood-vessels,  ve.  Between  the  costal  anlage  and  the  mesothelium  is  a 
layer  of  embryonic  connective  tissue,  the  cells  of  which  are  more  crowded  toward 


322  STUDY  OF  PIG  EMBRYOS. 

the  mesothelium,  so  that  we  may  say  that  two  layers  of  mesenchyma  are  already 
imperfectly  differentiated  within  the  rib.  The  denser  layer  next  the  mesothelium 
is  destined  to  become  still  more  marked  and  to  transform  itself  into  the  connective- 
tissue  layer  of  the  peritoneum.  With  the  overlying  mesothelium  it  develops  into 
the  peritoneal  membrane  of  descriptive  anatomy. 

Sagittal  Section  through  the  Right  Lung  and  Kidney  (Fig.  218). — The  lungs 
occupy  a  position  in  the  upper  part  of  the  figure  and  are  easily  recognized  by  the 
conspicuous-  entodermal  bronchi,  bro,  which  resemble  in  microscopic  structure  the 
bronchi  of  earlier  stages.  The  branches  are  widely  separated  from  one  another  by 
the  voluminous  mesenchyma  of  the  organ.  .  The  lung  is  covered  by  mesothelium, 
msth,  and  projects  into  the  pleural  cavity,  Pleu.c,  which  is  lined  by  a  continuation 
of  the  mesothelium  of  the  lung  itself.  The  pleural  cavity  can  be  followed  down- 
ward past  the  Wolffian  body,  W.b',  and  liver,  and  from  there  past  the  genital 
gland,  Gen,  and  so  on  to  the  lowest  part  of  the  abdominal  cavity,  Ab.cce.  The 
pleural  cavity  at  this  stage  is  entirely  separated  from  the  pericardial,  but  it  is 
still  directly  continuous  with  the  abdominal  cavity.  On  the  ventral  side  (in  the 
figure,  to  the  right)  of  the  pleural  cavity  are  the  great  veins,  the  common  cardinal, 
C.C,  descending  from  above,  and  the  ductus  venosus,  Du.v,  rising  from  below.  The 
pleural  cavity  is  separated  from  the  common  cardinal  by  a  lamina  of  the  mesoderm, 
x,  and  from  the  ductus  venosus  by  a  similar  but  thinner  lamina,  Y.  Both  laminae 
are  bounded  on  the  tdeural  side  by  the  mesothelium,  and  on  the  venous  side  by 
the  endothelium  of  the  vessel.  The  opening  of  the  veins  into  the  right  auricle, 
Au.d,  does  not  appear  in^this  section,  though  a  small  bit  of  the  left  valve,  v.s, 
which  guards  this  opening  is  shown.  The  Wolffian  body  is  divided  into  two  parts, 
an  upper,  W.b',  on  a  level  with  the  liver,  and  a  lower,  W.b",  toward  the  pelvic 
end  of  the  abdomen.  The  lower  part  is  larger  than  the  upper.  The  two  parts  are 
separated  from  one  another  chiefly  by  the  mesonephric  vein,  V.msn,  which  is  the 
principal  vessel  to  take  the  blood  from  the  Wolffian  body.  It  delivers  the  blood  to 
the  lower  end  of  the  vena  cava  inferior.  The  separation  of  the  two  parts  of  the 
Wolffian  body  is,  however,  further  accented  by  the  position  of  the  genital  gland, 
Gen,  and  of  the  kidney,  Ki.  The  structure  of  the  latter  organ  does  not  differ  much 
from  that  of  earlier  stages,  but  the  diameter  of  the  tubules  has  increased,  and  there 
has  been  an  advance  in  the  differentiation  of  the  convoluted  tubules  and  of  the 
glomeruli.  The  genital  gland  (testis)  is  remarkable  for  its  large  size.  It  is  covered 
by  a  layer  of  mesothelium,  underneath  which  is  a  rather  broad  layer  of  elongated 


FIG.  218. — PIG,  20.0  MM.    SAGITTAL  SERIES  60,  SECTION  213. 

Ab.cce,  Abdominal  ccelom.  All.vi,  Mesothelial  villi  of  the  allantois.  Au.d,  Right  auricle,  bro,  Entodermal 
bronchus.  C.C,  Common  cardinal.  Cae',  Cce",  Ccelom.  Diaph',  Diaph",  Diaphragm.  Du.v,  Ductus 
venosus.  G.bl,  Gall-bladder.  Gen,  Genital  gland.  In',  In",  In'",  Intestine.  Ki,  Kidney.  Li,  Liver. 
Mes,  Mesenchyma.  msth,  Pleural  mesothelium.  P.cce,  Pleural  ccelom.  Pleu.c,  Pleural  cavity.  Ve.  hep, 
Ve.hep',  Hepatic  veins.  V.msn,  Vena  mesonephrica.  v.s,  Valvula  sinistra.  W.b',  W.b",  Wolffian  body. 
x,  Partition  separating  the  pleural  cavity  from  the  duct  of  Cuvier.  Y,  Partition  separating  the  pleural  cavity 
from  the  ductus  venosus.  X  22  diams. 


STUDY  'OF  SECTIONS  OF  EMBRYO  OF  20  MM. 


323 


C.C.  v.s,     P.cce.     Au.d. 


Pleu.c 


Diaph" 


Ki. 


W.b 


All.vi. 


324  STUDY  OF  PIG  EMBRYOS. 

mesenchymal  cells,  the  anlage  of  the  tunica  albuginea.  The  interior  of  the  organ 
contains  a  number  of  contorted  epithelioid  cords  of  cells  in  which  there  are  a  cer- 
tain number  of  so-called  primitive  ova,  cells  which  are  distinguished  by  their  larger 
size,  rounded  form,  greater  transparency,  and  spherical  nuclei.  The  bands  of  cells 
are  known  as  the  sexual  cords,  and  they  are  separated  from  one  another  by  loose 
mesenchymal  tissue.  The  cords  frequently  anastomose  with  one  another.  They  are 
the  solid  anlages  of  the  seminiferous  tubules.  The  question  of  the  origin  of  these 
cords  has  been  much  debated,  but  cannot  be  considered  as  yet  settled.  As  to  both 
the  origin  and  the  ultimate  fate  of  the  primitive  ova  in  the  mammalian  testis 
we  have  only  incomplete  information.  The  cords  remain  solid  throughout  embryonic 
life,  not  acquiring  a  central  cavity  until  after  birth.  The  liver  is  a  very  voluminous 
organ  permeated  everywhere  by  sinusoidal  blood-vessels  which  offer  the  greatest 
possible  variety  in  size.  In  the  figure  only  the  larger  of  these  blood-vessels  have  been 
drawn  in,  A  large  proportion  of  the  smaller  sinusoids  are  crowded  with  nucleated 
red  blood-corpuscles,  the  nuclei  of  which  are  small  and  deeply  stained;  hence  each 
cluster  of  corpuscles  stands  out  as  a  darker  spot  in  the  liver,  for  the  liver  cells 
themselves  stain  lightly  and  have  nuclei  which,  though  three  or  four  times  the  size 
of  the  nuclei  of  the  blood-corpuscles,  yet  appear  relatively  pale  in  the  stained 
specimen.  The  blood-corpuscles  which  form  the  clusters  in  the  liver  differ  some- 
what from  those  in  the  active  circulation,  for  they  are  smaller,  and  show  less  of  the 
characteristic  hemoglobin  color.  It  has  been  demonstrated  that  the  liver  at  this 
stage  furnishes  sites  for  the  multiplication  of  the  blood-corpuscles,  and  the  clusters, 
which  are  so  conspicuous  in  the  organ,  correspond  not  to  blood-corpuscles  in  active 
circulation,  but  rather  to  corpuscles  which  have  found  a  lodging-place  in  the  liver 
and  are  there  proliferating.  Our  knowledge  of  the  blood-forming  function  of  the 
embryonic  liver  is  imperfect.  Above  the  liver  is  the  septum  transversum  or  dia- 
phragm, Diaph' ',.  Diaph" ',  which  is  formed  chiefly  by  mesenchyma.  On  the  lower  side 
of  the  liver  is  another  broad  accumulation  of  mesenchyma,  Mes,  in  which  is  lodged 
the  gall-bladder,  a  small  section,  G.bl,  of  the  entodermal  lining  of  which  is  included. 
The  intestine,  In',  In",  In"',  is  cut  several  times,  because  at  this  stage  the  intes- 
tinal canal  forms  several  coils  in  the  abdominal  cavity  below  the  liver  and  on  the 
ventral  side  of  the  Wolffian  bodies.  Below  the  intestines  appear  the  curious  meso- 
thelial  villi,  All.vi,  of  the  allantois  (compare  page  253).  At  this  stage  the  villi 
are  little  more  than  large  vesicles  of  mesothelium,  which  contain  in  their  interior 
some  coagulum  and  a  very  few  mesenchymal  cells,  associated  with  which  are  a  few 
fibers — whether  true  connective-tissue  fibers  or  not  is  undetermined.  The  mesothe- 
lium of  the  villi  is  a  very  thin  layer  of  flattened  cells. 

Frontal  Sections  of  the  Head. — The  three  sections  to  be  described  are  from 
a  special  series  of  the  head.  The  plane  of  the  series  was  made  as  nearly  as  possi- 
ble transverse  and  at  right  angles  to  the  plane  of  the  roof  of  the  mouth.  They 
illustrate  some  important  points  in  regard  to  the  development  of  the  facial  region 
and  of  the  fore-brain.  In  all  of  the  sections  the  differentiation  of  the  mesoderm 


FRONTAL  SECTIONS  OF  HEAD,  EMBRYO  OF  20  MM. 


325 


around  the  brain  is  clearly  demonstrated.  The  pia  mater  is  very  distinct.  In  those 
parts  of  the  sections  where  the  brain-wall  is  cut  obliquely,  it  can  be  distinguished 
only  by  a  somewhat  careful  observation,  as  the  tissues  of  the  pia  mater  and  of 
the  brain  overlap.  All  about  the  brain  is  the  broad  zone  of  the  arachnoid  (Figs. 
220  and  221,  arach),  easily  distinguishable  even  with  a  low  power  by  its  light  colora- 
tion. It  consists  of  widely  separated  cells  connected  together  by  very  distinct  pro- 
cesses, and  is  permeated  by  a  number  of  small  blood-vessels  running  in  various  direc- 
tions through  the  layer.  Its  external  boundary  is  now  very  distinct,  being  marked 
by  a  layer,  Sk,  of  somewhat  crowded,  elongated  cells  which  merge  on  the  side 


Sept. 


Jk.o.  - 


Max. 


nas.tb. 


nax.tb. 


MX. sup. 


Mdb. 


FIG.  219. — PIG,  20.0  MM.    FRONTAL  SECTION  or  HEAD.     SERIES  40,  SECTION  68. 

H,  Cerebral  hemisphere.  Jk.o,  Jakobson's  organ.  Max,  Maxillary  process,  max.tb,  Maxillo-turbinal  fold. 
Mdb,  Mandible.  MX. sup,  Superior  maxillary  nerve,  nas.tb,  Naso-turbinal  fold.  Sept,  Nasal  septum.  Sk, 
Mesenchymal  anlage  of  the  dura  mater  and  skull.  X  18  diams. 

toward  the  ectoderm  into  the  general  surrounding  mesenchyma.  Out  of  this 
denser  layer  (Figs.  219  and  220,  Sk)  arise  both  dura  mater  and  the  membrane 
bones  of  the  skull. 

Section  through  the  Anterior  Part  of  the  Snout  (Fig.  219). — On  the  dorsal  side 
appear  the  two  cerebral  hemispheres,  H,  cut  separately  and  each  showing  the 
cavity  of  its  lateral  ventricle.  On  the  ventral  side  the  mandible,  Mdb,  is  cut 
separately  and  is  separated  by  the  oral  fissure  from  the  rest  of  the  section.  The 
maxillary  processes,  Max,  are  large,  and  each  is  furnished  with  an  inward  prolonga- 
tion extending  toward  the  median  line.  From  the  oral  fissure  there  extend  upward 


326 


STUDY  OF  PIG  EMBRYOS. 


two  irregular  cavities,  the  nasal  chambers.  The  two  cavities  are  separated  from 
one  another  by  a  broad  mass  of  tissue,  the  nasal  septum,  the  ventral  edge  of  which 
at  this  stage  forms  a  portion  of  the  roof  of  the  mouth-cavity.  In  the  center  of  the 
nasal  septum  is  a  broad  band,  Sept,  of  denser  mesenchymal  tissue,  the  anlage  of 
the  cartilaginous  septum  of  the  nose.  On  either  side  of  the  nasal  septum  is  the 
irregularly  shaped  nasal  cavity,  which  opens  into  the  mouth  between  the  ven- 
tral edge  of  the  nasal  septum  and  the  inner  edge  of  the  maxillary  process.  The 

arach.  '  Sk. 


Mdb. 


FIG.  220. — PIG,  20.0  MM.    FRONTAL  SECTION  OF  HEAD.     SERIES  40,  SECTION  84. 

arach,  Arachnoid  membrane.  H,  Cerebral  hemispheres.  Max.tb,  Maxillo-turbinal  fold.  Mdb,  Mandible.  MX. 
sup,  Superior  maxillary  nerve.  Nas.tb,  Naso-turbinal  fold.  N.olf,  Olfactory  nerve.  Sept,  Cartilaginous 
septum  of  the  nose.  Sk,  Mesenchymal  anlage  of  the  dura  mater  and  skull.  Ton,  Tongue.  X  18  diams. 

medial  side  of  each  nasal  cavity  is  comparatively  regular,  but-  the  external  side 
shows  two  prominences,  each  of  which  is  formed  by  a  mass  of  mesenchymal  tissue 
covered  by  epithelium.  The  upper  of  these  projections,  nas.tb,  is  the  anlage  of 
the  naso-turbinal  fold,  and  the  lower,  max.tb,  the  anlage  of  the  maxillo-turbinal 
fold.  In  the  nasal  septum  itself  are  two  oval  rings  of  epithelium,  sections  of 
Jakobson's  organs.  This  organ  is  an  evagination  of  the  epithelial  lining  of  the 
nasal  cavity,  which  opens  anteriorly  and  extends  backward  some  distance  in  the 
nasal  septum.  In  the  maxillary  process  may  be  observed  the  superior  maxillary 
nerve,  Mx.sup.  The  number  of  cells  in  the  nerve  has  increased,  and  consequently 


FRONTAL  SECTIONS  OF  HEAD,  EMBRYO  OF  20  MM.  327 

the  division  of  the  nerve-fibers  into  distinct  bundles  has  become  more  marked 
as  compared  with  the  pig  embryo  of  12  mm. 

Section  through  the  Middle  of  the  Snout  (Fig.  220). — The  relations  are  very 
similar  to  those  described  in  the  previous  section,  so  that  it  will  suffice  to  note  the 
three  most  important  differences:  first,  the  absence  of  Jakobson's  organ;  second, 
the  appearance  of  the  tongue,  Ton,  and  third,  of  the  olfactory  nerve,  N.olf.  The 
tongue  is  a  protuberance  attached  to  the  lower  jaw,  Mdb.  Its  connection  with 
the  jaw  is  rather  narrow  and  corresponds  to  the  frenum.  The  tongue  extends 
upward  between  the  maxillary  processes  until  it  is  almost  or  quite  in  contact  with 
the  lower  edge  of  the  nasal  septum.  It  is  formed  by  a  somewhat  dense  mass  of 
tissue  in  which  there  is  no  very  evident  histological  differentiation,  and  is  covered 
by  a  layer  of  epithelium  of  moderate  thickness  and  which  is  probably  entirely 
derived  from  the  entoderm,  for  the  tongue  first  appears  as  a  small  median  pro- 
tuberance on  the  ventral  floor  of  the  pharynx,  between  the  first  gill-pouches. 
The  olfactory  nerve,  N.olf,  can  be  seen  joining  the  lower  part  of  the  inner  side  of 
the  brain-wall  and  extending  down  toward  the  nasal  cavity  and  branching.  Under 
the  part  of  the  nerve  near  the  brain-wall  numerous  cells  are  mingled  with  the 
fibers,  and  by  their  crowding  render  the  nerve  conspicuous  in  stained  sections. 
The  fibers  of  the  olfactory  nerve  differ  from  all  other  nerve-fibers  known '  in  ver- 
tebrates. They  arise  as  prolongations  of  certain  of  the  epithelial  cells  of  the 
olfactory  region  of  the  nose  and  grow  from  these  cells  into  the  brain,  where  they 
have  their  termination  in  the  glomeruli  of  the  olfactory  bulb.  All  other  nerve- 
fibers  arise  from  nerve-cells  either  of  the  central  nervous  system  or  of  the  gan- 
glia. Morphologically,  therefore,  the  olfactory  nerve  takes  a  unique  place,  and 
is  not  directly  comparable  with  any  other  nerve  of  the  brain.  The  cells  which 
accumulate  in  the  course  of  the  olfactory  nerve  do  not,  so  far  as  known,  have  any 
direct  share  in  the  production  of  the  nerve-fibers;  nor  do  they  result  in  the 
formation  of  the  medullary  sheaths,  as  they  do  in  other  nerves,  the  olfactory 
nerve-fibers  remaining  naked,  as  it  is  termed,  throughout  life.. 

Section  through  the.  Fore-brain  and  Eyes  (Fig.  221). — The  section  passes  behind 
the  nasal  cavities,  no  part  of  which  is  shown.  The  maxillary  and  mandibular 
processes  are  united  and  the  pharynx,  Ph,  appears  as  a  closed  cavity.  On  the 
dorsal  side  of  the  section  the  fore-brain  stands  out  conspicuously,  both  from  its 
dark  staining  and  from  being  surrounded  by  the  lightly  stained  broad  zone  of  the 
arachnoid,  arach.  The  cavity  of  the  fore-brain  has  two  lateral  expansions,  L.V, 
the  lateral  ventricles,  which  extend  outward  and  upward.  The  walls,  H,  of  the 
lateral  ventricles  are  much  thinner  than  the  walls  of  the  lower  part  of  the  fore- 
brain  and  are  the  anlages  of  the  cerebral  hemispheres.  In  the  median  plane  the 
hemispheres  include  between  themselves  a  partition,  Fx,  of  mesodermic  tissue 
which  may  be  designated  as  the  embryonic  falx,  since  within  it,  though  considerably 
later,  the  adult  falx  will  be  differentiat  d.  In  the  adult,  the  falx  appears  as  a 
prolongation  of  the  dura  mater.  From  the  bottom  of  the  falx  there  extends  on 


328 


STUDY  OF  PIG  EMBRYOS. 


each  side  a  fold,  Plx,  which  projects  into  the  cavity  of  the  lateral  ventricle.  This 
fold  contains  in  its  interior  a  prolongation  of  the  mesodermic  tissue  of  the  falx, 
and  it  is  covered  by  a  continuation  of  the  wall  of  the  hemispheres.  The  covering 
layer  of  the  fold  is  much  thinner  than  any  other  portion  of  the  brain-wall  shown 
in  the  section,  and  shows  no  differentiation  into  layers.  It  retains  throughout 


Fx.       ec.gl. 


H. 


L.V. 


Plx. 


C.str. 


m.rec.sup. 
m.retr.b. 


hy.gl. 


art. 


FIG.  221. — PIG,  20  MM.     FRONTAL  SECTION  or  HEAD.     SERIES  40,  SECTION  123. 

arach,  Arachnoid  zone,  art,  Lingual  arteries.  C.str,  Corpus  striatum.  ec.gl,  Ectoglia.  Fx,  Falx  cerebri.  H, 
Cerebral  hemisphere,  hy.gl,  Hyoglossal  muscle.  L,  Lens.  L.V,  Lateral  ventricle.  Mk,  Meckel's  car- 
tilage, m.rec.sup,  Musculus  rectus  superior,  m.retr.b,  Musculus  retractor  bulbi.  m.r.lat,  Musculus  rectus 
lateralis  (cf.  text).  Mx.i,  Inferior  maxillary  nerve. '  Ph,  Pharynx.  Plx,  Plexus  choroideus  lateralis.  Ret, 
Retina.  Sk,  Anlage  of  membranous  skull.  Ton,  Tongue,  x,  Unidentified  structure.  X  18  diams. 

life  an  epithelial  character  and  is  already  to  be  termed  ependyma.  The  ependyma 
of  the  two  folds  is  connected  across  the  median  line,  and  it  forms  the  median 
dorsal  boundary  of  the  cavity  of  the  fore-brain.  The  two  folds  are  the  anlages 
of  the  lateral  choroid  plexus.  They  are  destined  to  grow  much  in  size  and  in 
complexity  of  form,  but  they  always  remain  morphologically  what  they  now  are, 


FRONTAL  SECTIONS  OF  HEAD,  EMBRYO  OF  20  MM.  329 

vascularized  mesenchyma  covered  by  ependyma.  The  choroid  plexus  protrudes 
into  the  cavity  of  the  brain  in  the  same  way  in  which  the  viscera-  may  be  said 
to  protrude  into  the  abdominal  cavity.  The  cavity  of  the  brain  is  bounded  by 
the  brain-wall  or  ependyma,  just  as  the  abdominal  cavity  is  bounded  by  the 
peritoneum.  The  vascular  tissue  of  the  choroid  plexus  is  outside  of  the  cavity 
of  the  brain,  in  the  same  way  that  the  tissue  of  the  kidney  is  outside  the  cavity 
of  the  abdomen.  Throughout  life  the  choroid  plexus  springs,  as  it  does  from  the 
start,  from  the  medial  wall  of  the  hemispheres,  and  it  is  only  at  that  point  that 
it  can  receive  its  blood-supply.  The  lateral  walls  of  the  hemispheres,  H,  gradually 
thicken  as  they  continue  ventralward,  and  on  the  ventral  side  of  the  brain  form 
in  part  the  lateral  boundary  of  the  medial  portion  of  the  brain-cavity,  as  an  especial 
thickening  of  the  brain-wall  which  projects  far  into  the  cavity.  The  thickening, 
C.str,  is  the  corpus  striatum.  Between  the  summit  of  the  corpus  striatum  and 
the  choroid  plexus  is  an  open  passage  through  which  we  may  pass  from  the 
median  portion  of  the  brain-cavity  into  the  lateral  ventricle,  L.V.  The  passage 
is  the  foramen  of  Munro,  which  we  learn  from  this  section  is  bounded  above 
by  the  choroid  plexus,  and  below  by  the  corpus  striatum.  On  the  dorsal  and 
middle  sides  of  the  hemispheres,  the  ectoglia,  ec.gl,  is  already  clearly  differentiated. 
There  is,  however,  at  this  stage,  no  clear  indication  of  the  cortex  cerebri,  al- 
though in  the  slightly  older  stages  it  will  begin  to  develop  by  the  accumulation  of 
neuroblasts  immediately  beneath  the  ectoglia.  The  notochord  does  not  appear 
between  the  brain  and  the  pharynx,  the  section  being  too  far  forward.  The  noto- 
chord stops  near  the  hypophysis.  The  eyes  are  not  cut  quite  symmetrically.  They 
show  the  lens,  L,  and  retina,  Ret,  clearly  and  the  left  eye  of  the  embryo  shows 
also  the  entrance  of  the  optic  nerve.  On  the  right  side  of  the  embryo,  near  the 
eye,  are  three  areas  which  are  somewhat  more  darkly  stained  than  the  surrounding 
mesenchyma.  These  are  the  anlages  of  the  muscles  of  the  eye.  They  have  not 
yet  been  studied  sufficiently  to  make  their  identification  certain,  but  it  seems  prob- 
able that  the  uppermost  of  these  anlages,  m.rec.sup,  is  the  rectus  superior,  that  the 
middle  one,  m.retr.b,  is  the  retractor  bulbi,  and  that  the  lowest  one,  m.r.lat,  is 
the  rectus  lateralis.  Until  a  reconstruction  is .  made  these  identifications  can  be 
recorded  as  tentative  only.  The  pharynx,  Ph,  appears  as  a  yoke-shaped  slit  lined 
throughout  by  entoderm.  From  its  median  ventral  floor  rises  the  great  mass  of 
the  tongue,  Ton,  over  which  the  dorsal  roof  of  the  pharynx  forms  a  closely  fitting 
arch.  A  portion  of  the  epithelium  of  the  tongue  is  loosened  from  the  underlying 
tissue,  probably  owing  to  defective  preservation.  Upon  the  lower  side  of  the 
tongue  extend  downward  the  anlages  of  the  hyoglossal  muscles,  hy.gl,  between 
which  are  situated  the  lingual  arteries,  art.  On  either  side,  in  the  part  of  the 
section  corresponding  to  the  mandible,  appears  Meckel's  cartilage,  Mk,  a  some- 
what conspicuous  and  easily  identified  structure,  owing  to  its  dark  staining.  MeckeVs 
cartilage  is  the  primitive  skeletal  element  of  the  mandibular  arch,  and  is  homol- 
ogous with  the  cartilaginous  jaw  of  the  lower  fishes.  It  is,  for  the  greater  part, 


330 


STUDY  OF  PIG  EMBRYOS. 


a  purely  embryonic  structure,  the  mandible  of  the  adult  being  a  secondary  bone. 
By  referring  to  figure  166  (pig,  10  mm.),  it  can  be  seen  that  the  mandibular  arch 
extends  upward  toward  the  otocyst  and  forms  the  boundary  of  the  first  gill-cleft, 
the  outer  division  of  which  becomes  the  meatus  auditorius  externus.  In  other 
words,  the  upper  portion  of  the  mandibular  arch  is  in  close  proximity  to  the 
otocyst  and  the  anlage  of  the  tympanum  or  middle  ear.  Meckel's  cartilage  is  a 


R 


.    N 


FIG.  222. — RABBIT  EMBRYO  OF  THIRTEEN  DAYS;  SECTION  OF  THE  EYE.; 

EC,  Epidermis.    L,  Lens,     mes,  Mesenchyma.     N,  Anlage  of  optic  nerve.     P,  Pigment  layer.    R,  Retina,     tu.v , 

Tunica  vasculosa  lentis. 

rod-like  structure  extending  the  entire  length  of  the  arch.  Its  upper  end  is,  there- 
fore, close  to  the  future  tympanum.  While  the  greater  part  of  Meckel's  cartilage 
disappears  during  later  development,  the  upper  end  persists  and  takes  a  direct 
share  in  the  formation  of  the  malleus.  A  little  outside  of  Meckel's  cartilage  in 
our  'section  is  the  inferior  maxillary  nerve,  Mx.i,  and  still  farther  lateralward  is  a 
small,  darkly  stained  body,  x,  which  has  not  yet  been  identified  with  certainty. 

Pig  Embryo  of  24  mm.    Study  of  Sections. 

Section  through  the  Eye   (Fig.    223). — In  the  pig  of    24  mm.  the  anlages  of    all 

the  parts  of  the  adult  eye  may  be  said   to  be  present,   with   the  exception  of  the 

pigment  layer  of  the  iris,  which  arises  somewhat  later  by  a  forward  growth  of  the 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  24  MM.  331 

retina  and  pigment  layer.  The  origin  of  the  retina  and  lens  is  illustrated  by  the 
chicken  embryo  (Figs.  153,  154),  and  in  a  more  advanced  stage  by  the  pig  of 
12  mm.  (Fig.  192).  There  is  added  here  figure  222,  from  a  section  of  the  eye 
of  a  rabbit  embryo  of  thirteen  days,  in  order  to  facilitate  the  comparison  between 
the  12  mm.  stage  and  the  24  mm.  stage  of  the  pig.  In  figure  222  the  ectoderm, 
EC,  forms  an  arch  over  the  eye  and  indicates  the  commencing  formation  of  the 
cornea,  the  layer  of  ectoderm  being  destined  to  become  the  external  epithelium 
of  the  cornea.  Between  the  lens  and  the  retina  there  has  been  an  ingrowth  of 
tissue  accompanied  by  blood-vessels,  which  forms  a  more  or  less  distinct  covering 
over  the  surface  of  the  lens  and  constitutes  the  so-called  tunica  vasculosa,  tu.v.  The 
space  between  the  retina  and  lens  will  increase  during  the  following  stages  and 
will  become  occupied  by  a  very  clear  tissue  containing  a  minimal  number  of  cells. 
This  "clear  tissue  is  the  commencement  of  the  vitreous  humor.  Between  the  lens 
and  the  overlying  ectoderm  the  mesenchyma  has  begun  to  penetrate.  This  mesen- 
chyma  will  ultimately  furnish  the  connective  tissue  of  the  cornea  and  of  the  iris. 
About  the  eyeball  as  yet  there  is  no  distinct  condensation  of  tissue  such  as  will 
appear  later  to  develop  the  anlages  of  the  choroid  and  scleral  coats  of  the  eyeball. 
In  the  pig  of  24  mm.  (Fig.  223)  we  encounter  a  marked  advance  in  the  differ- 
entiation of  all  parts  of  the  eye.  Above  and  below  the  eye  the  anlages  of  the 
eyelids,  L.sup,  L.inf,  have  appeared.  The  anlage  is  at  this  stage  merely  a  projecting 
fold  of  the  ectoderm  filled  with  mesenchyma  and  extending  a  short  distance  over 
the  projecting  cornea.  -The  folds  will  continue  to  grow  until  the  eyelids  meet  in 
the  middle  of  the  eye,  covering  it  completely.  The  ectoderm  of  the  two  lids  where 
they  meet  unites.  The  union  of  the  two  lids  occurs  in  all  mammals,  and  in  some 
cases  they  do  not  separate  again  until  after  birth,  in  which  case  the  animals  are 
said  to  be  "born  blind."  The  ectoderm,  EC,  of  the  cornea  describes  a  wide,  high 
arch,  underneath  which  is  a  broad  band  of  embryonic  connective  tissue,  corn, 
which  forms  the  main  thickness  of  the  cornea.  Between  the  connective  tissue 
of  the  cornea  and  the  anterior  surface  of  the  lens  is  a  clear  space,  an.ch,  which  we 
can  identify  as  the  anterior  chamber  of  the  eye,  which  in  the  adult  is  filled  only 
with  the  aqueous  humor.  On  the  corneal  side  the  anterior  chamber  is  bounded 
by  a  distinct  layer  of  cells,  Ep,  the  internal  epithelium  of  the  cornea.  This  layer 
is,  however,  formed  from  the  mesenchyma,  the  cells  of  which  develop  into  the 
internal  epithelioid  covering  of  the  cornea.  At  the  upper  and  lower  edge  of  the 
cornea  there  is  a  separate  forward  growth,  Ir,  of  the  connective  tissue  between 
the  cornea  and  the  lens.  It  is  the  anlage  of  the  connective-tissue  layer  of  the  iris. 
In  later  stages  it  will  grow  still  farther  over  the  lens  from  all  sides,  leaving  a  cen- 
tral opening,  the  pupil,  and  it  will  acquire  a  special  pigmented  layer  on  its  side 
nearest  the  lens.  At  the  base  of  the  iris  anlage  is  a  small  blood-vessel,  Schl,  which 
is  commonly  designated  in  the  adult  as  the  canal  of  Schlemm.  The  retina  has 
increased  in  thickness  and  is  closely  covered  by  a  pigment  layer,  Pig.  The  separa- 
tion which  appears  on  the  inner  side  of  the  eyeball  between  the  retina  and  pig- 


332 


STUDY  OF  PIG  EMBRYOS. 


ment  layer  in  figure  223  is  probably  artificial,  the  result  of  shrinkage  during  the 
preservation  of  the  specimen.  At  its  outer  edge  the  retina,  Ret,  suddenly  thins  out 
and  passes  over  into  the  external  pigment  layer,  which  is  heavily  loaded  with  dark, 
yniform,  pigment  granules,  especially  crowded  together  on  the  side  of  the  layer 


L.sup. 


N.op. 


Vit. 


Schl. 


tu.v. 


Ret. 


L.inf. 


FIG.  223. — PIG,  24.0  MM.     TRANSVERSE  SERIES  62,  SECTION  428. 

an.ch,  Anterior  chamber  of  eye.  corn,  Corneal  mesoderm.  EC,  Ectoderm  (epidermis).  Ep,  Inner  epithelium 
of  cornea.  Ir,  Mesodermal  anlage  of  iris.  L',  Outer  layer  of  lens.  L",  Inner  layer  of  lens.  L.inf,  Inferior 
eyelid.  L.sup,  Superior  eyelid.  ^.3,  Oculo-motor  nerve.  N.op,  Optic  nerve.  Pig,  Pigment  layer.  Ret, 
Retina.  Schl,  Canal  of  Schlemm.  tu.v,  Tunica  vasculosa  lentis.  Vit,  Vitreous  humor.  X  50  diams. 

nearest  the  retina.  In  later  stages  the  pigment  layer  grows  forward  on  the  inner 
side  of  the  iris,  making  a  fold,  so  that  the  iris  is  covered  on  the  inside  by  a 
double  layer  of  pigmented  epithelium,  the  uvea.  The  retina  resembles  closely  in 
structure  the  brain-wall  in  an  early  stage,  for  it  has  on  its  outer  surface  a  thin 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  24  MM.  333 

layer  corresponding  to  the  ectoglia,  and  within  a  broad,  nucleated  zone.  The 
mitotic  figures  are  found  only  next  to  the  surface  of  the  retina  nearest  the  pig- 
ment layer.  Since  the  space  between  the  pigment  layer"  and  the  retina  corresponds 
to  the  cavity  of  the  brain,  it  is  evident  that  the  position  of  the  mitotic  figures 
is  homologous  with  their  position  in  the  medullary  wall  elsewhere.  The  section 
of  the  lens  clearly  reveals  its  vesicular  structure.  The  external  wall  of  the  lens 
vesicle,  L',  is  a  comparatively  thin  epithelial  layer  which  stains  quite  readily  and 
therefore  stands  out  clearly  in  the  section.  Toward  the  edges  of  the  lens  the  outer 
layer  slightly  thickens  and  then-  passes  over  quite  abruptly  into  the  inner  layer  of 
the  vesicle,  L" ',  which  is  very  thick  and  constitutes  by  far  the  greater  part  of  the 
bulk  of  the  organ  and  gives  to  the  lens  its  characteristic  shape.  The  outer  and 
inner  walls  of  the  lens  are  in  close  contact  so  that  there  is  no  actual  cavity  present. 
The  epithelial  cells  of  the  inner  wall  have  elongated  enormously,  so  much  that 
they  might  perhaps  already  be.  termed  "fibers."  Each  cell  is  supposed  to  extend 
through  the  entire  thickness  of  the  inner  wall.  The  nuclei  are  placed  somewhat 
irregularly  in  the  middle  portion  of  the  long  -cells  so  that  they  constitute  a  more 
or  less  distinct  band  in  the  section.  Toward  the  edge  of  the  lens  the  nuclear  band 
becomes  more  distinct,  and  where  the  inner  wall  merges  into  the  outer,  the  band 
becomes  narrow  and  the  nuclei  are  much  crowded  together.  The  nuclei  of  the  lens 
fibers  are  oval,  being  slightly  elongated  in  the  same  direction  as  the  fibers,  and 
each  nucleus  contains  usually  a  distinct  nucleolus.  Between  the  lens  and  the  retina 
is  the  vitreous  humor,  Vit,  which  has  become  quite  voluminous.  It  contains  a  few 
mesenchymal  cells  and  a  few  small  blood-vessels,  and  when  examined  with  a  high 
power  it  is,  seen  to  be  permeated  by  a  fine  network  which  is  probably  to  be 
interpreted  as  a  modification  of  the  protoplasmic  threads  of  the  mesenc.hyma.  There 
are  also  a  very  few  cells  of  rounded  form  and  distinct  outline,  with  a  single  small 
granular  nucleus,  which  are  probably  leucocytes.  Against  the  surface  of  the  lens 
there  is  a  delicate  homogeneous  hyaloid  membrane,  which  can  usually  be  better 
seen  where  by  shrinkage  it  has  been  loosened  from  the  surface  of  the  lens,  as 
is  apt  to  occur.  Against  the  hyaloid  membrane  are  a  number  of  small  blood-ves- 
sels, more  numerous  than  those  elsewhere  in  the  vitreous  humor,  and  forming  a 
fairly  distinct  vascular  membrane  around  the  lens.  The  membrane,  tu.v,  is  called 
the  tunica-  vasculosa  lentis.  The  blood-vessels  of  the  vitreous  humor  are  chiefly, 
possibly  at  this  stage  exclusively,  branches  of  the  central  artery  of  the  retina.  The 
artery  enters  the  eye  through  the  optic  nerve,  and  sends  branches  throughout  the 
vitreous  humor.  The  space  originally  occupied  in  the  humor  by  the  stem  of  the 
central  artery  persists,  and  is  called  the  hyaloid  canal.  The  muscles  of  the  eye 
are  already  differentiated,  but  their  relations  cannot  be  properly  understood  without 
a  reconstruction. 

Median  Sagittal  Section  (Fig.  224). — The  section  figured  is  very  nearly  median 
for  the  region  of  the  head,  but  in  the  body  it  passes  to  the  left  of  the  median 
plane.  The  area  occupied  in  the  section  by  the  neck  and  head  of  the  embryo  is 


334 


Epen 


Ven.IV 


Md.ob 


Te 


Plx.IV. 


STUDY  OF  PIG  EMBRYOS. 
Cbl.  A.  M.b. 


Ar.hab. 


Cce. 


W.b 


T". 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  24  MM.  335 

almost  as  great  as  that  occupied  by  the  rest  of  the  body.  The  great  size  of  the 
head  at  this  stage  is  characteristic.  Attention  is  especially  directed  to  the  sharp 
angle  which  the  medulla  oblongata,  Md.ob,  makes  with  the  spinal  cord,  Sp.c,  and 
to  the  very  great  bend  formed  by  the  floor  of  the  mid-brain,  Ar.hab,  in  conse- 
quence of  which  the  floor  of  the  hind-brain  above  the  medulla  oblongata  and  the 
floor  of  the  fore-brain  are  brought  quite  close  together  and  run  in  almost  parallel 
directions.  The  cavity  of  the  brain  is  very  large.  Its  walls  in  the  median  plane 
are,  for  the  most  part,  thin.  From  the  roof  of  the  diencephalon,  Dien,  there  runs 
off  a  small  evagination,  Ephys,  a  shallow  pocket  or  diverticulum  of  the  medullary 
wall.  It  is  the  anlage  of  the  epiphysis  or  pineal  organ  of  the  adult.  It  is  an 
important  landmark  in  the  topography  of  the  brain,  for  its  position  is  always  at 
the  extreme  posterior  limit  of  the  fore-brain.  In  the  wall  of  the  mid-brain,  behind 
the  epiphysis,  for  some  distance  the  ectoglia  shows  considerable  thickening  and 
contains  a  very  large  number  of  nerve-fibers  running  transversely.  They  constitute 
the  posterior  commissure,  which  morphologically  belongs  to  the  mid-brain.  In  later 
stages  the  opening  of  the  epiphysis  and  the  anterior  boundary  of  the  posterior 
commissure  are  separated  by  a  narrow  band  of  ependyma.  Immediately  in  front 
of  the  epiphysis-,  close  to  the  external  surface  of  the  medullary  wall,  is  another 
tract  of  nerve-fibers  which  is  very  small  and  is  known  as  the  superior  commissure. 
The  superior  commissure  develops  much  later  than  the  posterior,  and  is  much 
smaller  in  size.  The  two  commissures  are  found  in  vertebrates  of  all  classes  and 
are  exceedingly  constant  anatomical  features  of  the  brain.  Anterior  to  the  epiphy- 
sis the  dorsal  roof  of  the  diencephalon  forms  a  broad  arch  which  descends  in 
the  figure  vertically  until  it  ends  in  a  small  inward  projection,  Fix,  of  the  brain- 
wall,  the  anlage  of  the  choroid  plexus.  Below  this  point  the  brain-wall  is  continued, 
forming  the  lamina  lerminalis.  It  then  makes  a  bend  almost  at  right  angles  and 
runs  in  a  horizontal  direction  toward  the  dorsal  side  of  the  embryo.  This  portion 
of  the  brain- wall  shows  a  considerable  thickening,  the  optic  chiasma.  Behind  the 
optic  chiasma  the  brain-wall  forms  a  short  evagination,  the  infundibular  gland,  which 
bends  over  so  as  •  to  lie  close  to  the  dorsal  side  of  the  hypophysis,  Hyp.  The 
hypophysis,  which  in  earlier  stages  appears  clearly  as  an  evagination  of  the  oral 


FIG.  224. — PIG,  24.0  MM.     SAGITTAL  SERIES  63,  SECTION  30. 

A,  Arachnoid  space,  in  this  specimen  containing  extravasated  blood.  A.Ao,  Arch  of  the  main  aorta.  All.ar, 
Allantoic  artery.  Ao,  Dorsal  aorta.  Ao.D,  End  of  dorsal  aorta.  Ar.hab,  Habenular  arch  (floor  of  mid- 
brain).  A. vert,  Vertebral  artery  joining  its  mate  to  form  the  basilar  artery.  Bro,  Main  bronchus  of  lung. 
bro,  Branch  bronchus  within  the  lung.  CW,  Cerebellum.  Coe,  Coelom.  Diaph,  Diaphragm.  Dien,  Dience- 
phalon. Duo,  Duodenum.  Epen,  Ependymal  roof  of  hind-brain.  Ephys,  Epiphysis.  G,  Spinal  ganglion. 
Hyp,  Hypophysis.  In,  Intestine.  Int.v,  Anlage  of  intervertebral  ligament.  La,  Lateral  wall  of  larynx.  Li, 
Liver.  Lu,  Lung.  M.b,  Mid-brain.  Md.ob,  Medulla  oblongata.  Nch,  Notochord.  (E,  (Esophagus. 
Pen,  Penis.  Ph,  Pharynx.  Plx,  Choroid  plexus  of  fore-brain.  Plx.IV,  Choroid  plexus  of  hind-brain. 
Sept,  Cartilaginous  nasal  septum.  Sp.c,  Spinal  cord.  Sp.ren,  Suprarenal  capsule.  St,  Stomach.  T',T", 
Tail.  Te,  Testis.  Ton,  Anterior  portion  of  tongue.  Umb,  Umbilical  cord.  Ve,  Cardinal  vein.  Ven, 
Ventricle  of  the  heart.  Ven. I V,  Fourth  ventricle,  or  cavity  of  the  hind-brain.  Vert,  Vertebra.  W.b,  Wolman 
body.  X  8  diams. 


336  STUDY  OF  PIG  EMBRYOS. 

epithelium  (Fig.  201),  is  now  entirely  separated  from  the  mouth,  and  is  an  epithe- 
lial vesicle  with  an  irregular  cavity.  The  epithelium  has  sent  out,  especially  on 
its  anterior  side,  a  number  of  solid  outgrowths.  The  infundibular  gland  and 
hypophysis  constitute  the  pituitary  body  of  the  adult.  They  are  surrounded  by 
loose  mesenchymal  tissue.  The  sella  turcica,  in  which  the  pituitary  body  of  the 
adult  is  lodged,  is  marked  out,  because  the  chondrification,  which  is  to  form  the 
sphenoidal  cartilages,  has  already  begun  about  these  structures.  The  sphenoidal 
cartilage  is  continuous,  on  the  one  hand,  with  that  of  the  nasal  septum,  Sept,  and, 
on  the  other,  with  that  of  the  vertebral  column,  Vert.  From  the  opening  of  the 
infundibular  gland  the  brain-wall  ascends  and  joins  the  habenular  arch,  where  it 
suddenly  thickens.  The  arch  forms  the  floor  of  the  mid-brain.  The"  roof  of  the 
mid-brain,  M.b,  is  quite  thin,  and  forms  the  large  arch  in  which  the  differentia- 
tion of  the  anterior  and  posterior  corpora  quadrigemina  is  not  yet  shown.  At  its 
posterior  boundary  the  wall  of  the  roof  of  the  mid-brain  bends  inward,  marking 
the  constriction  of  the  so-called  isthmus.  We  now  reach  the  cavity,  Ven.IV,  or 
fourth  ventricle,  of  the  hind-brain.  This  cavity  is  subdivided  into  an  anterior  and 
a  posterior  portion.  The  boundary  is  marked  on  the  dorsal  side  by  the  inward 
projection  of  the  ependymal  roof  of  the  ventricle  to  form  the  choroid  plexus,  Plx. 
IV,  and  on  the  ventral  side  by  the  angle  formed  by  the  union  of  the  medulla 
oblongata,  Md.ob,  with  the  vertical  peduncles  of  the  brain.  The  peduncles,  con- 
tinuing upward,  join  the  habenular  arch.  In  front  of  the  choroid  plexus  the  arch- 
ing brain-wall,  Cbl,  represents  the  median  anlage  of  the  cerebellum.  The  lateral 
portions  of  the  cerebellum  are  much  thicker.  Behind  the  choroid  plexus  the  roof, 
Epen,  of  the  fourth  ventricle  is  very  thin.  The  medulla  oblongata,  Md.ob,  is  a 
thick  mass  of  tissue  which  passes  over  abruptly  into  the  spinal  cord.  The  spinal 
cord  is  cut,  as  a  whole,  somewhat  obliquely.  In  its  upper  part,  where  the  reference 
line,  Sp.c,  is  placed,  the  section  is  almost  exactly  median,  and  shows,  therefore,  the 
floor-plate  or  raphe  of  the  spinal  cord.  In  front  of  the  cord  is  the  vertebral  artery, 
A. vert,  which  joins  its  fellow  to  form  the  basilar  artery  which  runs  in  the  median 
line  the  entire  length  of  the  hind-brain.  The  vertebral  column  is  in  the  cartilagi- 
nous stage.  It  is  an  absolutely  continuous  uninterrupted  rod  of  cartilage  which 
merges  at  the  neck  with  the  cartilaginous  skull.  The  entire  continuous  cartilaginous 
structure  is  termed  the  chondrostyle,  for  the  study  of  which  comparison  with  the 
neighboring  sections  is  indispensable.  Out  of  it  both  the  cartilaginous  skull  and 
the  vertebrae  are  differentiated.  More  or  less  nearly  in  the  center  of  the  chondro- 
style are  found  the  remnants  of  the  notochord,  which,  however,  never  extends 
anterior  to  the  pituitary  body,  Hyp.  The  division  of  the  chondrostyle  into  separate 
vertebrae  is  indicated  by  the  segmental  flexures"  of  the  notochord  and  by  the  com- 
mencing differentiation  of  the  intervertebral  ligaments.  The  space  occupied  by  the 
notochord  expands  in  the  region  corresponding  to  the  division  between  each  two 
vertebrae.  The  notochord  in  the  intervertebral  expansions  is  expanded  and  partly 
degenerated,  forming  an  enlarged  mass  of  irregular  strands  of  cells,  which  becomes 


STUDY  OF  SECTIONS  OF  EMBRYO  OF  24  MM.  337 

the  nucleus  pulposus  of  the  adult.  From  each  such  mass  goes  off  a  narrow  exten- 
sion of  the  notochord,  through  what  is  to  become  the  body  of  the  vertebra.  Some- 
times this  extension  is  continuous  with  the  intervertebral  portions  of  the  notochord, 
but  more  usually  it  forms  a  series  of  isolated  fragments,  for  the  notochord  in  the 
parts  corresponding  to  the  bodies  of  the  vertebrae  is  already  in  process  of  resorption. 
The  diameter  of  the  chondrostyle  is  nearly  uniform  in  the  vertebral  region,  but 
is  a  little  smaller  in  the  part  corresponding  to  each  body  of  a  vertebra  and  a  little 
wider  in  the  parts  corresponding  to  the  intervertebral  ligaments.  The  cartilage  of 
the  body  of  the  vertebra  continues  past  the  intervertebral  expansion  of  the  noto- 
chordal  cavity,  but  the  external  portion  of  the  chondrostyle  opposite  each  such 
expansion  exhibits  a  modification  of  its  cells,  for  they  have  become  lengthened  out 
in  a  direction  parallel  with  the  vertebral  axis.  The  tissue  thus  produced  is  the 
anlage  of  the  intervertebral  ligaments.  The  mouth  and  pharynx,  Ph,  form  a  nar- 
row cavity,  the  floor  of  which  is  constituted  by  the  tongue,  Ton,  the  tip  of  which 
has  already  become  free.  The  surface  of  the  tongue  forms  a  long  arch,  at  the 
posterior  end  of  which  lies  the  epiglottis,  a  projecting  fold  of  tissue  which  covers 
the  opening  of  the  trachea.  The  side  of  the  trachea  is  marked  by  the  longitudi- 
nal fold,  La,  which  separates  the  trachea  proper  from  the  upper  end  of  the  ceso- 
phagus,  (E.  At  the  upper  end  of  the  oesophagus  there  is  a  small  dorsal  diverticu- 
lum.  If  the  reference  line  (E  were  continued  a  short  distance  past  the  oesophagus,  it 
would  lead  to  the  section  of  the  main  aorta.  A  little  lower  down  is  the  section  of 
the  arch  of  the  aorta,  A.Ao.  The  heart  shows  chiefly  its  large  ventricle,  Ven. 
The  section  is  not  favorable  for  an  exhibition  of  its  structure  or  for  that  of  the 
lungs,  Lu.  It  does,  however, — since  in  this  part  of  the  embryo  the  section  passes  to  . 
one 'side  of  the  median  plane, — show  the  main  bronchus,  Bro,  coming  off  from  the 
trachea  to  the  lung,  and  some  of  the  smaller  entodermal  bronchial  branches,  bro, 
in  the  lung  itself.  The  heart  and  lung  are  separated  from  the  abdominal  cavity 
by  the  diaphragm,  Diaph.  It  is  only  to  the  dorsal  part  of  this  diaphragm  that  the 
liver,  Li,  is  attached.  In  earlier  stages  the  liver  is  connected  with  the  whole  of  , 
the  diaphragm  (septum  transversum) .  We  now  have  a  portion  of  the  diaphragm 
without  connection  with  the  liver.  Below  the  liver  is  the  section  of  the  stomach, 
St,  the  entoderm  of  which  is  cut  twice.  Below  the  stomach  lies  the  duodenum, 
Duo,  extending  from  the  dorsal  side  of  the  embryo  and  running  toward  the  um- 
bilicus. At  the  dorsal  end  of  the  duodenum  is  a  group  or  cluster  of  darkly  stained 
cells,  marking  the  position  of  the  anlage  of  the  pancreas.  Below  the  duodenum  the 
loops  of  the  intestine,  In,  are  cut  repeatedly.  On  the  dorsal  side  of  these  loops  is 
the  section  of  the  genital  gland,  in  this  specimen,  testis,  Te.  Dorsalward  from 
the  genital  gland  is  the  complicated  anlage  of  the  suprarenal  capsule,  Sp.ren,  which 
is  really  a  double  organ,  having  one  part  derived  from  the  sympathetic  nervous 
system  and  another  from  a  modification  of  mesenchymal  cells.  In  a  sagittal  series 
the  connection  of  the  anlage  with  the  sympathetic  nervous  chain  of  the  abdomen 
can  be  readily  made  out.  In  the  anlage  the  nerve-fibers  and  the  sympathetic  cells 


338  STUDY  OF  PIG  EMBRYOS. 

are  irregularly  distributed,  although  the  cells  are  more  or  less  grouped  together. 
The  sympathetic  tissue  constitutes  the  dorsal  part  of  the  anlage  and  gives  rise  to 
the  so-called  medulla  of  the  adult  organ.  The  ventral  portion  of  the  anlage,  as 
seen  in  the  section,  consists  of  bands  or  cords  of  cells  separated  from  one  another 
by  venous  sinusoids.  The  cells  are  much  more  closely  compacted  in  this  portion 
of  the  anlage  than  in  the  sympathetic,  and  they  are  further  distinguished  by  hav- 
ing nuclei  which  stain  much  less  deeply.  The  cords  of  cells,  here  seen,  develop 
into  the  cortex  of  the  adult  organ.  The  fate  of  the  medulla  or  .sympathetic  portion 
of  the  suprarenal  in  man  is  not  known.  The  section  passes  through  the  side  of 
the  allantois,  and,  therefore,  shows  only  one  of  the  lateral  arteries,  All.ar,  but  the 
allantois  still  bears  a  number  of  degenerating  mesothelial  villi  (compare  page  253). 
At  the  pelvic  end  of  the  abdomen  a  small  bit  of  the  Wolffian  body,  W.b,  is 
displayed. 


CHAPTER  VII. 
STUDY  OF  THE  UTERUS  AND  THE  FETAL  APPENDAGES  OF  MAN. 

Histology  of  the  Uterus. 

In  most  mammals  the  uterus  is  double.  This  is  the  case  in  the  pig,  the 
rabbit,  and  the  mouse,  the  three  species  which  furnish  material  for  the  practical 
study  as  planned  in  this  book.  In  these  animals  each  uterus  is  a  long,  more  or 
less  cylindrical  tube.  In  primates  the  double  uterus  exists  only  during  very  early 
embryonic  stages,  after  which  the  two  are  found  united  into  a  single  median 
uterus.  The  mammalian  uterus  is  always  lined  by  a  mucous  membrane,  con- 
sisting of  a  superficial  epithelium  which  forms  glands,  and  of  a  deeper  layer  of 
reticulate  connective  tissue,  in  which  there  are  lymph  spaces,  nerves,  and  a  fairly 
abundant  blood  supply.  The  mucous  membrane  is  subject  to  very  marked  periodic 
changes  in  structure.  It  is  enclosed  by  the  muscular  layers  of  the  organ,  the 
muscle-fibers  being  of  the  smooth  type.  In  animals  with  double  uteri  the  muscle- 
fibers  form  two  distinct  layers,  an  inner  circular  and  an  outer  longitudinal  layer. 
In  the  primate  'uterus  the  disposition  of  the  fibers  is  far  more  complicated,  and 
the  two  distinct  layers  cannot  be  identified.  The  surface  of  the  uterus,  wherever 
it  is  free,  is  covered  by  a  layer  of  peritoneum  which  consists  of  a  layer  of  flattened 
epithelial  cells  and  a  thin  underlying  layer  of  fibrillar  connective  tissue. 

The  human  uterus  at  birth  has  a  mucosa  which  is  about  0.2  mm.  thick. 
The  mucosa  is  soft,  pale  gray  or  reddish  gray  in  color;  it  consists  of  a  covering 
of  ciliated  epithelium  and  a  connective-tissue  layer.  It  is  without  glands,  the 
glands  not  appearing  usually  until  the  third  or  fourth  year,  and  developing  very 
slowly  up  to  the  age  of  puberty. 

The  development  of  the  human  uterine  glands  is  accompanied  by  remarkable 
and  complex  changes  of  Ahe  epithelium.*  The  adult  glands,  as  shown  by  figure  225, 
are  much  branched,  and  the  branches  occasionally  anastomose  with  one  another. 
The  model,  from  which  the  figure  is  taken,  demonstrates  that  the  conception  of  the 
character  of  the  uterine  glands,  which  has  hitherto  prevailed,  is  very  inadequate. 

Menstruation. 

The  function  of  menstruation  involves  great  changes  in  the  mucosa  of  the 
body  of  the  uterus.  We  distinguish  three  periods:  (i)  tumefaction  of  the  mucosa, 
with  accompanying  structural  changes,  taking  five  days,  or,  according  to  Hensen, 

*  Unpublished  investigations  by  C.  A.  Hedblom,  to  whom  I  am  indebted  for  the  privilege  of  inserting  figure  225. 

339 


340 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


ten  days;    (2)   menstruation  proper,   about  four  days;    (3)   restoration  of  the  resting 
mucosa,  about    seven    days.      The    times    given    are  approximate  only.      The  whole 

cycle  of  changes  covers  about  sixteen 
days.  Since  the  monthly  period  is  about 
four  weeks,  the  period  of  rest,  as  thus 
calculated,  is  only  about  twelve  days. 

1.  Tumefaction. — A    few     days     be- 
fore    the     menstrual     flow     the     mucosa 
gradually   thickens;    the    surface    becomes 
irregular;   the  openings  of  the  glands  lie 
in    depressions.       The     connective-tissue 
cells    are    increased    in    number,    and    it 
is     said     by     some    authors    in   size,    but 
the    increase    in    size    is    doubtful;    the 
number    of    round    cells    increases;     the 
glands    expand   and   become   more   irreg- 
ular in  their  course;    a   short  time  before 
hemorrhage     begins,     the     blood-vessels, 
especially    the    capillaries    and   veins,   be- 
come   greatly    distended.       We   must   as- 
sume    that     the     connective-tissue     cells 
proliferate,    but   we    have    no  satisfactory 
observations  upon  their  division.     It  was 
formerly     asserted     that     the     menstrual 
decidua    contains    decidual    cells,    but    in 
all  the  specimens  the  author  has  studied 
there  were  none  present. 

2.  Menstruation. — When  the  changes 
just  described  are  completed,  the  decidua 
menstrualis     is     fully     formed,     and    its 
partial       disintegration       begins..        The 
process    commences    with    an    infiltration 


of    blood    into    the    subepithelial   tissues. 
This    infiltration  has    hitherto    been  com- 
monly  explained   as   due    to    the  rupture 
FIG.  225.— UTERINE  GLAND  OF  A  VIRGIN  OF  EIGHTEEN     of  the  capillaries;    but  as  no  ruptures  at 
YEARS,   WITH  A  PORTION  OF  THE  SURFACE  EPI-     this   period  have  been  observed,  we  may 

THELIUM.       WAX     RECONSTRUCTION    BY    C.    A.      .      ,,  •>     .,•  ,•  , 

,.  uistly    regard    this    explanation    as    mad- 

HEDBLOM.     X  50  diams.  J        * 

missible,   and   account  for  the  infiltration 

per  diapedesin.  It  lasts  for  a  day  or  two,  and  is  apparently  the  immediate  cause 
of  a  very  rapid  molecular  disintegration  of  the  superficial  layers  of  the  mucosa, 
which  in  consequence  are  lost;  the  superficial  blood-vessels  are  now  exposed,  and, 


MENSTRUATION.  341 

by  rupturing,  cause  the  well-known  hemorrhagia  of  menstruation.  By  the  dis- 
appearance of  its  upper  portion  the  mucosa  is  left  without  any  lining  epithelium 
and  is  very  much  (and  abruptly)  reduced  in  thickness.  Its  surface  is  formed  by 
connective  tissue  and  exposed  blood-vessels. 

3.  Restoration  of  the  Mucosa. — At  the  close  of  menstruation  the  mucosa  is 
2  or  3  mm.  thick;  the  regeneration  of  the  lost  layers  begins  promptly  and  is  com- 
pleted in  a  variable  time,  probably  from  five  to  ten  days.  The  hyperemia  rapidly 
disappears;  the  extra vasated  blood-corpuscles  are  partly  resorbed,  partly  cast  off; 
the  spindle-cell  network  grows  upward,  while  from  the  cylinder  epithelium  of  the 
glands  young  cells  grow  and  spread  up  and  out  so  as  to  produce  a  new  epithelial 
covering;  new  subepithelial  capillaries  appear.  The  details  of  these  changes  are 
imperfectly  known;  they  effect  the  return  of  the  mucosa  to  its  resting-stage. 

Decidua  Menstrualis. — Specimens  from  the  first  day  of  menstruation  are  the 
most  instructive.  They  should  be  preserved  in  Zenker's  fluid;  sections  may  be 
made  perpendicular  to  the  decidual  surface  from  blocks  10  to  15  mm.  cube,  and 
stained  with  alum  hematoxylin  and  eosin.  The  use  of  Mallory's  triple  connective- 
tissue  stain  will  demonstrate  the  fibrillar  tissue  in  the  decidua  and  the  very  large 
amount  of  the  same  in  the  muscularis. 

The  accompanying  illustration  (Fig.  226)  is  from  a  uterus  in  active  menstrua- 
tion. The  decidual  membrane  is  from  i.i  to  1.3  mm.  thick;  its  surface  is 
irregularly  tumefied;  the  gland  openings  lie  for  the  most  part  in  the  depressions. 
In  the  cavity  of  the  uterus  there  was  a  small  blood-clot.  The  demarcation 
between  the  decidua  and  the  muscularis  is  sharp.  The  upper  fourth,  d,  of  the 
decidua  is  broken  down  and  very  much  disintegrated;  its  cells  stain  less  readily 
than  those  of  the.  deep  portion  of  the  membrane;  the  tissue  is  divided  into  numer- 
ous more  or  less  separate  small  masses.  Some  of  the  blood-vessels  are  ruptured. 
The  superficial  epithelium,  ep,  is  loosened  everywhere;  in  places  fragments  of  it 
have  fallen  off,  and  in  some  parts  it  is  gone  altogether;  it  stains  readily  with 
alum  hematoxylin,  differing  in  this  respect  from  the  underlying  connective  tissue. 
The  deeper  layer  of  the  decidua  is  dense  with  crowded  well-stained  cells,  which 
lie  in  groups  and  are  probably  proliferated  connective-tissue  cells.  They  have 
small  oval  or  elongated  darkly  stained  nuclei,  with  very  small  granular  protoplas- 
matic bodies.  There  is  no  indication  of  any  enlargement  of  the  cells,  such  as 
occurs  in  the  production  of  true  " decidual"  cells.  There  are  very  few  leucocytes. 
The  glands  are  enlarged  somewhat,  and  are  lined  by  a  normal  cylinder  epithelium, 
which  offers  no  obvious  change  as  compared  with  that  of  the  glands  of  the  resting 
uterus. 

•     .  •• 

The  Pregnant  Uterus:  the  Two  Stages. 

When   the  ovum   implants  itself  in   the   uterine   wall,   it  becomes  covered  by  a 

growth    of    the    mucous    membrane    or    decidua    which    we  know    as    the    decidua 

reflexa.      For  an   account  of  this  process   see  pages    124  to  127,  where  also  proper 


342 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


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THE,PREGNANT  UTERUS. 


343 


definitions  of  the  terms  decidua  reflexa,  serotina,  and  vera  are  given.  -As  the 
ovum  increases  in  size  the  decidua  reflexa  also  increases,  and  gradually  becomes 
thinner  and  thinner,  until  it  ultimately  disappears.  The  exact  date  of  its  disap- 
pearance is  not  known  ;  it  falls  somewhere  within  the  fifth  month.  Accordingly, 
we  may  divide  the  period  of  pregnancy  into  two  phases  or  stages,  each  com- 
prising about  half  of  the  whole  period.  During  the  first  stage  the  decidual 
reflexa  is  present  (Fig.  227);  during  the  second  stage  it  is  absent,  so  that  the 
chorion  l.aeve  comes  into  direct  contact  with  the  decidua  vera.  In  the  following 
sections  a  typical  uterus  of  the  first  and  second  stages'  each  is  described. 


FIG.  227.— HUMAN  UTERUS  ABOUT  ONE  MONTH  PREGNANT.    NATURAL  SIZE.     (THE  UTERUS  HAS  BEEN  OPENED 

BY  AN  INCISION  ALONG  ITS  MEDIAN  LlNE,  SO  AS  TO  DISPLAY   THE   SMALL  OVAL  BAG  FORMED  BY  THE  DECIDUA 

REFLEXA.) 

Human  Uterus  Three  Months  Pregnant. 

The  uterus  measures  about  3^  inches  in  transverse  diameter,  and  shows 
well-marked  venous  sinuses  on  its  external  surface.  It  should  be  opened  by  a 
crucial  incision  on  the  anterior  side;  its  walls  will -be  found  about  an  inch  or  more 
in  thickness;  it  contains  a  grayish  red  bag  (decidua  reflexa),  which  nearly  fills 
the  cavity  of  the  uterus  and  encloses  the  embryo,  so  that  upon  opening  the  womb 
we  do  not  encounter  the  fetus  directly.  The  inner  bag  has  a  smooth  surface, 
but  shows  a  few  small  pores;  it  is  without  blood-vessels  and  is  attached  to  the 
dorsal  wall  of  the  uterus.  The  inner  surface  of  the  uterus  shows  a  rich  network 


344  HUMAN  UTERUS  AND  FETAL  APPENDAGES. 

of  blood-vessels,  many  of  which  are  large,  irregular  sinuses.  The  uterine  walls 
consist  of  an  outer  muscular  layer,  and  an  inner  decidual  layer;  the  latter  takes  up 
nearly  half  the  thickness  of  the  wall,  and  is  known  as  the  decidua  vera.  Com- 
parison with  the  seventh  month  uterus  shows  that  the  proportion  of  the  layers 
changes,  because  during  gestation  the  muscular  layer  increases  and  the  decidual 
layer  diminishes  in  thickness.  The  inner  bag,  when  opened,  shows  the  large 
cavity  in  which  .the  embryo  lies  floating  in  amniotic  fluid.  The  bag  is  formed  by 
three  very  distinct  membranes,  of  which  the  outermost,  the  decidua  reflexa,  is 
opaque  and  the  thickest;  the  two  inner  ones  are  thin  and  transparent;  the  inner- 
most is  the  delicate  amnion;  the  middle  membrane  is  the  chorion,  and  is  quite 
distinct  from  -both  the  amnion  and  reflexa;  to  the  latter  it  is  connected  by  a 
number  of  small  branching  villi  scattered  at  some  distance  from  one  another  over 
the  surface;  the  villi  adhere  firmly  to  the  reflexa  by  their  tips.  The  embryo 
(Fig.  109)  resembles  a  child  in  its  general  appearance;  the  length  of  the  head 
and  rump  together  is  nearly  8  cm.,  and  the  head  is  approximately  equal  in  bulk 
to  the  rump.  The  umbilical  cord  is  from  5  to  7  mm.  in  diameter  and  usually 
about  12  cm.  long.  From  its  distal  end  the  blood-vessels  spread  out  over  the 
placental  area,  and  around  the  edge  of  the  area  rises  the  decidua  reflexa,  which 
does  not  extend  on  to  the  placenta.  Floating  in  the  amniotic  fluid  is  a  pear- 
shaped  vesicle,  the  yolk-sac,  which  is  about  7  mm.  in  diameter;  it  has  a  fine 
network  of  blood-vessels  upon  its  surface,  and  is  connected  at  its  pointed  end  with 
a  long,  slender  pedicle,  the  yolk-stalk,  which  runs  to  the  placental  end  of  the 
umbilical  cord,  there  enters  the  cord  itself,  and  runs  through  its  entire  length  to  its 
attachment  to  one  of  the  coils  of  the  intestine  of  the  embryo.*  Over  the  whole 
of  the  placental  area  the  chorion  gives  off  large  villous  trunks,  each  of  which  has 
numerous  branches,  with  ramifications  of  the  fetal  vessels;  the  villi  fill  a  space 
about  i  cm.  wide  between  the  membrane  of  the  chorion  frondosum  and  the 
surface  of  the  uterine  decidua  serotina,  to  which  the  tips  of  some  of  the  villi 
are  attached.  With  care  the  villi  may  be  separated  from  the  decidua,  which  is 
seen,  when  it  is  thus  uncovered,  to  be  cavernous;. the  caverns  are  rounded  in  form 
and  part  of  them  may  be  followed,  on  the  one  hand,  until  they  connect  with  the 
blood  sinuses  of  the  uterus,  and,  on  the  other,  until  they  open  into  the  intervillous 
spaces,  which  therefore  receive  a  direct  supply  of  blood  from  the  mother. 

The  principal  difference  to  be  noted  between  the  uterus  before  and  that  after 
the  fifth  month  in  the  relations  of  parts  is  the  presence  or  absence  of  the  decidua 
reflexa  as  a  distinct  membrane.  Duping  the  fourth  month  the  reflexa  stretches 
as  the  membranes  expand,  and  becomes  thinner  and  thinner  until  by  the  end  of 
the  fourth  month  it  is  as  delicate  and  transparent  as  the  chorion  and  lies  close 
against  the  decidua  vera. 


*  At  this  stage  a  large  part  of  the  yolk-stalk  within  the  umbilical  cord  has  degenerated  and  usually  disap- 
peared by  resorption. 


THE  PREGNANT  UTERUS.  345 

Human  Uterus  Seven  Months  Pregnant. 

If  we  examine  a  pregnant  uterus  at  any  time  during  the  sixth  to  ninth  month 
of  gestation,  we  find  essentially  the  same  relations  of  the  parts — the  most  marked 
difference  being  in  the  size  of  the  uterus,  which  increases  with  the  duration  of 
gestation,  to  correspond  to  the  growth  of  the  fetus.  A  description  of  a  uterus 
seven  months  after  conception  will  suffice,  therefore,  for  our  present  purpose. 

Such  a  uterus  is  a  large,  rounded  bag  with  muscular  walls,  and  measures 
7  or  8  inches  in  diameter.  Examined  externally,  it  is  remarkable  especially  for  the 
numerous  large  sinus-like  blood-vessels;  its  surface  is  smooth;  the  texture  of  the 
walls  is  firm  to  the  touch,  but  the  walls  yield  to  pressure,  so  that  the  position  of 
the  child  can  be  felt.  As  the  placenta  is  situated  normally  upon  the  dorsal  side, 
it  is  usual  to  open  the  uterus  by  a  crucial  incision  of  the  ventral  wall.  The  walls 
are  about  one  half  of  an  inch  thick,  sometimes  more,  sometimes  less,  and  as  soon 
as  they  are  cut  open  •  we  enter  at  once  into  the  cavity  of  the  uterus  containing  the 
fetus  and  nearly  a  pint  of  serous  liquid — the  amniotic  fluid.  The  fetus  normally 
lies  on  one  side,  has  the  head  bent  forward,  the  arms  crossed  over  the  chest,  the 
thighs  drawn  against  the  abdomen,  and  the  legs  crossed  (compare  Fig.  in).  It 
resembles  closely  the  child  at  birth,  but  is  smaller;  its  head  is,  relatively  to  the 
size  of  the  body,  larger;  the  abdomen  is  more  protuberant,  and  the  limbs  propor- 
tionately smaller.  The  inner  surface  of  the  uterus  is  smooth  and  glistening;  if  it  is 
touched  with  the  finger,  it  is  found  to  be  covered  by  a  thin  but  rather  tough  mem- 
brane, called  the  amniqn,  which  is  only  loosely  attached.  Examination  of  the  uter- 
ine wall,  where  it  has  been  cut  through,  shows  that  its  thickness  is  formed  princi- 
pally by  the  muscular  layer,  which  is  made  up  by  numerous  laminae  of  fibers, 
between  which  are  the  large  and  crowded  blood  sinuses,  similar  to  those  distin- 
guishable on  the  external  surface  of  the  uterus.  About  one  fifth  or  less  of  the 
wall  inside  the  muscularis  has  a  different  texture  and  can  be  partly  peeled  off  as 
two  distinct  membranes,  the  innermost  of  which  is  the  amnion  already  mentioned, 
and  the  outer  is  the  chorion  united  with  the  decidua.  The  amnion  and  chorion 
are  appendages  of  the  embryo;  the  decidua  is  the  modified  mucous  membrane  of 
the.  uterus.  Let  us  return  to  the  embryo.  From  its  abdomen  there  springs  a  long, 
whitish  cord,  known  as  the  umbilical  cord;  it  is  usually  .from  about  one  third  to 
one  half  an  inch  in  diameter  and  40  cm.  long,  but  its  dimensions  are  extremely 
variable;  it  always  shows  a  spiral  twist,  and  contains  three  large  blood-vessels,  two 
arteries  and  one  vein,  all  of  which  can  be  distinguished  through  the  translucent 
tissue.  The  distal  end  of  the  cord  is  attached  to  the  wall  (placenta)  of  the  uterus 
usually  near  the  middle  of  the  dorsal  side  of  the  organ.  It  is  easily  seen  that  the 
blood-vessels  of  the  umbilical  cord  radiate  out  from  its  end  over  the  surface  of  the 
uterus  underneath  the  amnion,  branching  as  they  go;  they  spread,  however,  only 
over  a  circumscribed  area,  the  placental,  where  the  wall  of  the  uterus  is  very 
much  thickened.  A  vertical  section  through  the  placental  area  -shows  that  the  am- 
nion and  chorion  are  widely  separated  from  T;he  decidua  and  muscularis  by  a 


346  HUMAN  UTERUS  AND  FETAL  APPENDAGES. 

spongy  mass  soaked  with  maternal  blood.  This  mass  consists  of  numerous  trees  of 
tissue,  which  spring  with  comparatively  thick  stems  from  the  chorion  and  branch 
again'  and  again.  In  these  stems  and  branches  are  to  be  found  the  final  ramifica- 
tions of  the  vessels  of  the  umbilical  cord;  the  trees  are  known  as  chorionic  or 
placental  mill.  Some  of  their  end-twigs  are  very  closely  attached  to  the  surface  of 
the  decidua.  In  the  center  of  the  placental  area  the  villi  form  a  mass  about  three 
fourths  of  an  inch  thick,  but  toward  the  edge  of  the  area  the  mass  gradually  thins' 
out  until  at  the  very  edge  the  chorion  and  decidua  come  into  immediate  contact. 
The  mass  of  villi,  together  with  the  overlying  portions  of  the  chorionic  and  am- 
niotic  membranes  and  the  underlying  portion  of  the  decidua,  constitutes  what  is 
known  as  the  placenta.  The  decidua  of  the  placental  area  is  called  the  decidua 
serotina;  the  chorion  of  the  placenta  is  known  as  the  chorion  frondosum.  When 
birth  takes  place,  the  whole  placenta  is  expelled  after  the  delivery  of  the  child;  the 
placenta  of  the  obstetrician  is,  therefore,  partly  of  fetal,  partly  of  maternal,  origin. 

Decidua  Vera  of  the  First  Stage  in  Section. 

Specimens  may  be  preserved  in  Zenker's  or  Tellyesnicky's  fluid,  or  they  may 
be  preserved  with  less  good  results  in  Miiller's  or  Parker's  fluid  or  in  picro-sulphuric 
acid.  Sections  may  be  made  of  the,  en  tire  wall  in  celloidin,  or,  if  it  is  desired  to 
get  thinner  sections,  in  paraffin,  in  which  case  it  is  advantageous  to  remove  as 
much  as  possible  of  the  muscular  coat  so  as  to  cut  only  the  decidual  membrane. 

The  following  description  is  based  upon  a  uterus  one  month  pregnant.  Figure 
228  was  obtained  from  a  vertical  section  of  the  decidua,  by  drawing  the  outlines 
of  the  glands  or  gland  spaces,  Gl,  and  by  dotting  the  entire  area  occupied 
by  the  connective  tissue.  The  blood-vessels  are  indicated  by  double  outlines. 
The  artery,  Art,  owing  to  its  spiral  course,  is  cut  repeatedly.  The  figure 
demonstrates  very  clearly  that  the  gland  cavities  are  so  arranged  that  the  decidua 
is  divided  into  an  upper  compact  layer,  Comp,  and  a  lower  cavernous  layer, 
Cav,  the  difference  being  due  to  the  size  and  number  of  the  gland  cavities.  The 
amount  of  epithelium  to  be  observed  at  this  stage  varies  greatly.  It  is  sometimes 
wholly  absent  from  the  surface,  in  other  cases  absent  or  present  in  patches.  In  the 
glands  the  epithelium  has.  undergone  many  modifications.  In  some  parts  the  original 
cylinder  epithelium  of  the  glands  is  well  preserved  in  patches,  and  such  patches 
of  epithelium  are  found  at  every  stage-  until  after  delivery.  It  has  been  observed 
that  these  patches  serve  to  regenerate  the  epithelium  of  the  glands,  and,  by  spreading 
from  the  glands  on  to  the  surface,  to  regenerate  also  the  epithelial  covering  of  the 
uterine  mucosa.  But  for  the  most  part  the  glandular  epithelium  is  considerably 
altered.  We  find  places  in  which  the  cells,  though  attached  to  the  surrounding 
connective  tissue,  are  separated  from  one  another  by  small  fissures.  In  other  places 
the  cells  are  a  little  larger  (Fig.  229),  each  for  the  most  part  cleft  from  its  fellow, 
and  some  of  them  loosened  from  the  wall  and  lying  free  in  the  cavity.  Apparently 
the  cells'  which  are  thus  freed  become  swollen,  probably  by  imbibition,  both  the 


DECIDUA   VERA   OF  FIRST  STAGE. 


347 


Comp. 


Cav. 


m    ,a: 


FIG.  228. — VERTICAL  SECTION  OF  A  HUMAN  UTERUS 
(DECIDUA  VERA),  ONE  MONTH  PREGNANT. 

Comp,  Compact  layer.  Cav,  Cavernous  layer.  D, 
Gland-duct.  Art,  Spiral  artery.  Cl,  Spaces 
occupied  by  epithelial  glands.  Muse,  Muscu- 
laris.  (For  clearness  all  the  glandular  epithe- 
lium has  been  omitted  from  the  drawing.) 


FIG.  229. — HUMAN  UTERUS,  ONE  MONTH  PREG- 
NANT. SECTION  OF  GLAND  FROM  THE  CAVERN- 
OUS LAYER,  WITH  THE  EPITHELIUM  PARTLY 
ADHERENT  TO  THE  WALLS.  X  445  diams. 


FIG.  230. — HUMAN  UTERUS,  ONE  MONTH  PREGNANT.     SECTION  OF  A  GLAND  FROM  THE  CAVERNOUS  LAYER  WITH 
THE  EPITHELIUM  LOOSENED  FROM  THE  WALLS.     THE  EPITHELIAL  CELLS  ARE  SWOLLEN. 


348 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


protoplasm  and  the  nuclei  becoming  enlarged  (Fig.  230).  The  cells  lie  separately 
and  almost  completely  fill  the  gland  cavity.  They  are  no  longer  cylindrical  in 
shape,  but  irregular.  Their  protoplasm  is  finely  granular  and  stains  rather  lightly. 
The  nuclei  are  rounded,  granular,  and  with  sharp  outlines.  In  somewhat  older 
stages  one  finds  the  cells,  replaced  by  a  granular  material.  The  obvious  interpreta- 
tion of  the  appearances  described  is  that  the  glandular  epithelium  is  breaking  down 
and  disintegrating,  or,  in  other  words,  passing  through  a  special  form  of  degenera- 
tion which  is  highly  characteristic.  In  later  stages  some  of  the  broken-down 


'  FIG.  231. — UTERUS  ONE  MONTH  PREGNANT;  PORTION  OF  THE  COMPACT  LAYER  OF  THE  DECIDUA  SEEN  IN 

VERTICAL  SECTION. 
Coagl,  Coagulum  upon  the  surface.     d,df,  Decidual  cells.     X  445  diams. 

material  forms  hyaloid  rounded  concretions,  which,  owing  to  their  deep  staining,  are 
somewhat  conspicuous.  The  concretions  usually  include  a  number  of  spherical 
vacuolcs. 

The  formation  of  decidual  cells  has  already  begun'  in  the  upper  or  compact 
layer  (Fig.  231).  They  are  modified  connective-tissue  cells,  which  have  grown  in 
size  and  altered  their  structure.  Their  bodies  stain  deeply-  with  eosin;  the  nuclei 
are  round,  oval,  slightly  irregular  in  shape,  coarsely  granular,  and  sharp  in  outline. 
The  cells  themselves,  though  irregular  and  variable  in  shape,  are  all  more  or  less 
provided  with  processes  running  off  in  various  directions.  Scattered  between  the 
cells  are  many  sections  of  the  processes.  Occasionally  it  may  be  seen  that  two 


DECIDUA  REFLEX  A   OF  FIRST  STAGE. 


349 


cells  are  connected.  Later  on  the  decidual  cells  acquire  smoother  and  more  rounded 
outlines,  and  appear  to  lose  altogether  their  connections  with  one  another.  In  the 
cavernous  layer  there  are  no  decidual  cells. 

Decidua  Reflexa  of  the  First  Stage. 

The  decidua  reflexa  may  be  preserved  in  Zenker's  fluid,  Parker's  fluid,  or  picro- 
sulphuric   acid.     It  should  be  hardened  with  the  portions  of    the  chorion  and  cho- 
rionic   villi  adherent  to  it.      It  may  be  im- 
bedded in  celloidin  and  the  sections  stained 
with    alum     hematoxylin    and    eosin,    with 
Beale's    carmine,  or  with  a  so-called  fibrin 
stain. 

As  stated  above  (page  343),  the  pres- 
ence of  the  decidua  reflexa  distinguishes 
the  first  stage  of  pregnancy  from  the 
second,  in  which  the  reflexa  is  absent, 
having  disappeared  by  degeneration  and 
absorption.  To  observe  this  process  of  the 
disappearance  of  the  reflexa,  membranes 
from  the  second  and  third  months  should 
be  examined. 

Section  of  Decidua  Reflexa  of  Two 
Months. — At  this  time  the  reflexa  starts 
from  the  edge  of  the  placental  area  as  a 
membrane  of  considerable  thickness,  but  it 
rapidly  thins  out,  the  very  thinnest  point 
being  opposite  the  placenta.  Examination 
of  sections  shows  that  the  entire  reflexa  is 
undergoing  degeneration  which  is  found  to 
be  more  advanced  the  more  remote  the 
part  examined  is  from  the  placenta.  The 
chorion  laeve  lies  very  near  the  reflexa,  being 
separated  only  by  the  villi,  which  are  already 
very  much  altered  by  degeneration.  In  the 
region  halfway  between  the  base  and  the 

apex  of  the  reflexa  the  tissue  (Fig.  232)  shows  only  vague  traces  of  its  original 
structure.  Only  here  and  there  can  a  distinct  cell  with  its  nucleus  be  made  out. 
Most  of  the  cells  have  broken  down  and  fused  into  irregular  hyaline  masses  with- 
out organization.  Ramifying  through  the  fused  detritus  appear  strands  and  lines, 
which  are  more  darkly  stained  by  both  carmine  and  hematoxylin.  On  account  of 
their  fibrous  appearance,  these  strands  are  often  spoken  of  as  fibrin,  although  they 
are  presumably  not  the  same  as  the  true  fibrin  from  the  blood.  The  fibrin  is  much 


FIG.  232. — SECTION  OF  HUMAN  DECIDUA  REFLEXA 
AT  Two  MONTHS. 


350  HUMAN  UTERUS  AND  FETAL  APPENDAGES. 

more  developed  upon  the  inner  or  chorionic  side  than  upon  the  outer  side  of  the 
reflexa.  On  the  inner  side  it  forms  a  dense  network,  which  fuses  with  the  degen- 
erated ectoderm  of  the  chorionic  villi  wherever  the  villi  are  in  contact  with  the  de- 
cidua.  It  also  ramifies  nearly  halfway  through  the  decidua,  the  ramifications  being 
followed  easily,  owing  to  the  dark  staining  of  the  substance.  Over  the  outside  of  the 
decidua  the  fibrin  forms  a  much  thinner  layer  or  may  be  only  indistinctly  formed. 

In  a  decidua  reflexa  of  three  months  the  conditions  are  essentially  the  same, 
except  that  the  degeneration  is  further  advanced  and  the  membrane  thinner. 
Traces  of  cellular  structure  are  still  more  vague  and  the  fibrin  is  more  developed. 
In  all  parts  of  the  membrane  there  appear  leucocytes  which  are  particularly 
numerous  and  conspicuous  in  the  neighborhood  of  the  placenta.  It  is  natural  to 
assume  that  they  are  concerned  in  the  resorption  of  the  reflexa.  There  is  an  inner 
thicker  layer  of  fibrin  and  a  thinner  outer  layer,  which  is  now  always  present  and 
distinct.  Between  these  two  layers  is  a  stratum  in  which  the  remains  of  the  cells 
may  be  seen.  Occasionally  there  is  an  appearance  which  suggests  surviving  de- 
cidual  cells,  and,  indeed,  in  sections  taken  from  parts  close  to  the  placenta  true 
decidual  cells  may  be  identified. 

The  origin  of  the  chorion  laeve  by  the  disappearance  of  its  villi  is  described 
on  page  367.  The  sections  of  the  decidua  reflexa  will  enable  the  student  to  see 
also  some  of  the  phases  of  the  degeneration  of  the  villi.  They  are  very  much 
altered.  Their  ectoderm  undergoes  a  hypertrophic  degeneration  and  becomes  hya- 
line tissue,  which  stains  darkly.  The  degenerated  ectoderm  of  adjacent  villi  fuses 
more  or  less  extensively.  The  mesoderm  of  the  villi  shows  a  partial  loss  of  its 
primitive  cellular  organization. 

Decidua  Vera  and  Chorion  Laeve  of  the  Second  Stage. 

Pieces  of  the  decidua  vera  of  from  six  to  nine  months  with  the  chorion  and 
amnion  carefully  preserved  in  situ  may  be  hardened  in  Miiller's  or  Tellyesnicky's 
fluid.  Blocks  half  an  inch  or  less  in  size  may  be  imbedded  in  celloidin,  and  sec- 
tions made  perpendicularly  to  the  surface  may  be  stained  with  alum  hematoxylin 
and  eosin,  or  with  Heidenhain's  hematoxylin  and  orange  G,  or  with  picro-carmine. 

The  decidua  reflexa  having  been  resorbed,  the  chorion  (Fig.  233,  Cho)  has 
come  into  contact  with  the  surface  of  the  uterus,  and  the  chorionic  epithelium,  c, 
is  closely  adherent  to  the  surface  of  the  decidua,  from  which  the  original  epithe- 
lium has  completely  disappeared.  The  amnion  is  loosely  connected  with  the 
chorion  by  a  few  strands  or  threads,  which  are  represented  in  the  figure  and  the 
nature  of  which  is  not  known.  Both  the  amnion.  Am,  and  the  chorion,  Cho, 
being  developed  from  the  original  somatopleure  (compare  page  82),  consist  of  a 
mesodermic  and  an  ectodermal  layer.  The  ectoderm  of  the  amnion  is  a  single 
layer  of  cuboidal  cells  placed  on  the  side  of  the  membrane  toward  the  embryo  and 
away  from  the  uterus.  The  ectoderm,  c,  of  the  chorion,  on  the  contrary,  is  next 
the  uterus.  Hence  it  will  be  noticed  that  the  mesodermic  layers  of  the  amnion 


DECIDUA   VERA  AND  CHORION  LMVE  OF  THE  SECOND  STAGE.      351 

and  chorion  are  adjacent.  Both  membranes  are  quite  thin.  The  decidua  is  a 
relatively  voluminous  membrane  containing  blood-vessels,  v,  which  for  the  sake  of 
distinctness  have  been  filled  in  with  black  in  the  drawing.  It  also  contains  a 
series  of  elongated  spaces,  which  represent  sections  of  the  glands.  These  spaces, 
gl,  are  present  only  in  the  inferior  half  of  the  decidua.  Owing  to  their  absence 
from  the  superior  half,  that  portion  has  a  more  compact  structure,  and  is,  there- 
fore, designated  as  the  compact  layer;  the  lower  portion,  being  broken  up  and 


c-:,v;K\^v-^^C^!s^^^C^^:-^i^;^^?rt^;»:V 

•%:£.  ^^'^^^^f^^?^^^^M^^^; 


,^^    ^        ^ 


FIG.  233.  —  HUMAN   UTERUS  ABOUT  SEVEN  MONTHS  PREGNANT.    VERTICAL   SECTION  OF  THE   DECIDUA  VERA 

WITH  THE   FETAL  MEMBRANES  IN  SITU. 

Am,  Amnion.      Cho,  Chorion.       c,  Chorionic  epithelium.      v,  Blood-vessel.       gl,  Glands.      muse,  Muscularis. 

X  40  diams. 


made  loose  in  texture  by  the  somewhat  numerous  gland  cavities,  is  called  the 
cavernous  layer,  the  caverns,  of  course,  corresponding  to  the  gland  spaces.  The 
gland  spaces  are  now  very  much  stretched  out,  a  condition  which  results  simply 
from  the  general  expansion  of  the  uterus  during  pregnancy.  In  the  gland  spaces 
appear  patches  of  epithelium  still  intact,  and  in  the  cavities  themselves  isolated 
cells  in  various  phases  of  degeneration  and  disintegration,  similar  to  the  phases 
which  may  be  observed  in  the  decidua  vera  of  one  month;  but  the  degeneration  is, 
on  the  whole,  considerably  more  advanced  than  in  the  early  stage.  Around 


352  HUMAN  UTERUS  AND  FETAL  APPENDAGES. 

some  of  the  larger  blood-vessels  there  is  connective  tissue  only  slightly  modified, 
and  the  original  structure  of  the  mucous  membrane  is  more  or  less,  but  not 
perfectly,  preserved  in  the  deep  portion  of  the  decidua.  The  majority  of  the 
cells,  especially  in  the  compact  layer,  have  grown  in  size  and  become  transformed 
into  true  decidual  cells.  In  the  ectoderm  of  the  chorion,  c,  the  cells  lie  two  or 
three  deep.  They  have  distinct  walls,  a  very  coarsely  granular  protoplasm,  and 
nuclei  which  stain  darkly.  By  these  characteristics  they  are  easily  distinguished 
from  the  neighboring  decidual  cells,  to  which,  however,  they  offer  a  slight  super- 
ficial resemblance.* 

The  Placenta  in  Situ. 

The  placenta  in  its  natural  position  in  the  uterus  follows  the  curvature  of  the 
uterine  walls,  hence  its  free  or  amniotic  surface  is  slightly  concave.  Its  decidual 
surface  is  strongly  convex.  It  is  thickest  in  the  center  and  thins  out  gradually 
toward  its  edge.  The  uterus  should  be  obtained  in  the  freshest  possible  condition 
and  be  opened  by  a  crucial  incision  on  the  ventral  side.  The  embryo  should 
then  be  removed,  fhe  umbilical  cord  cut  through,  care  being  taken  to  bring  as 
little  pressure  as  possible  on  the  uterus  or  the  placenta,  and  the  whole  organ 
placed  in  the  preservative,  which  should  be  either  Tellyesnicky's  or  Miiller's  fluid. 
In  view  of  the  large  size  of  the  organ,  it  is  very  necessary  to  use  large  quantities 
of  the  preserving  fluid,  and  this  fluid  must  be  changed  several  times  in  order  to 
insure  good  histological  preservation.  When  the  hardening  is  completed,  columns 
about  one-half  inch  square  may  be  cut  out  so  as  to  pass  vertically  from  the  inner 
to  the  outer  surface  of  the  placenta,  preserving  the  amniotic  and  chorionic  mem- 
branes in  place.  The  blocks  are  to  be  imbedded  in  celloidin  and  ought  to 
remain  at  least  three  days  in  thin  and  three  days  in  thick  celloidin,  so  as  to  insure 
a  thorough  penetration  of  the  imbedding  material  into  the  intervillous  spaces. 
Make  the  sections  so  that  they  pass  vertically  through  the  placenta.  Stain  with 
alum  hematoxylin  and  eosin. 

Placenta  at  Seven  Months. — A  section  made  according  to  the  method  just 
described  is  'represented  in  figure  234.  The  thin  amnion,  Am,  covers  the  upper 
(or  inner)  surface  of  the  chorionic  membrane,  Cho.  This  membrane  is. separated 
from  the  decidua,  D,  by  a  dense  forest  of  villi,  of  which  innumerable  sections 
appear.  In  younger  placentas  the  distance  between  the  chorion  and  the  decidua 
is  considerably  less,  and  the  number  of  sections  of  villi  is  smaller,  but  the  average 
size  of  those  sections  larger.  In  the  present  specimen  the  distance  between  the 
chorion  and  the  decidua  is  nearly  twice  as  great  as  the  diameter  of  the  muscular 
coat,  Me,  of  the  uterus.  The  ends  of  some  of  the  villi  touch  the  decidual  tissue, 
and  are  imbedded  in  it.  Their  imbedded  ends  are  without  ,  covering  epithelium, 
but  their  connective  tissue  is  immediately  surrounded  by  hyaline  substance  which 

*  It  should  perhaps  be  noted  that  in  some  comparatively  recent  text-books  the  chorionic  ectoderm  has  been 
described  as  the  decidua  reflexa,  an  error  which  is  much  to  be  regretted. 


THE  PLACENTA  IN  SITU. 


353 


FIG.  234. — HUMAN  PLACENTA  IN  SITU,  ABOUT  SEVEN  MONTHS.    VERTICAL  SECTION. 

Am,  Amnion.  Cho,  Chorion.  Vi,  Villous  trunk,  vi,  Sections  of  villi  in  the  substance  of  the  placenta.  D',D", 
Decidua  serotina.  Me,  Muscularis.  Ve,  Uterine  artery,  opening  into  the  placenta;  the  fetal  blood-vessels  are 
drawn  black;  the  maternal  blood-vessels  are  white;  the  chorionic  tissue  is  stippled,  except  the  canalized  fibrin, 
which  is  shaded  by  lines.  The  remnants  of  the  gland  cavities  in  the  decidua  are  stippled  dark.  X  6  diams. 

23 


354 


HUM Atf  UTERUS  AND  FETAL  APPENDAGES. 


is  probably  degenerated  epithelium.  The  decidua  serotina  is  plainly  divided  into 
an  -upper  compact,  D',  and  a  lower  cavernous  layer,  D".  The  section  figured 
passes  through  an  arterial  vessel,  Ve,  which  makes  an  abrupt  turn  so  as  to  dis- 
charge its  blood  into  the  intervillous  spaces. 

The  histological  structure  of  all  the  parts  should  be  carefully  studied.  (As 
regards  the  structure  of  the  amnion,  see  page  370.) 

The  chorion  consists  of  two  layers,  the  outer  ectodermic  and  inner  mesodermic. 
Over  the  chorionic  membrane  proper  the  ectoderm  offers  a  great  variety  of  appear- 
ances. In  some  places  it  may  be  seen  to  have  still  its  primitive  organization,  a 
single  inner  layer  of  distinct  cells  and  an  outer  syncytial  layer,  more  or  less  similar 


Am. 


-  J'TTTJ;  *  »--:rV»'  3'<fcv'-v*~  ^^  v31  C  *""'!  '"  ** 

__•_-'  -*•    .--s-^  <3--<:r~*&>*      -...,-•»*..,.     — — ^-, «*>v.    ^-V...^.^--^"""1 

"**»  "  "  •**     •""      -*M"*:'-  -  '•'•"'   "         '    :'-*-;— ••".-•''•J-—   '  "•'        *         "*       ~'  --- 


ZFib- 


FIG.-  235. — HUMAN  PLACENTAL  CHORION  AND  AMNION  OF  THE  FIFTH  MONTH. 

Ep,  Amniotic  epithelium.     Am,  Amnion.     Str,    Stroma.     Fib,  Fibrillar  layer.     Fbr,  Fibrin  layer,     c,  Chorionic 
cellular  layer  of  ectoderm.     Vi,  Chorionic  villi.      X  71  diams. 


to  those  represented  in  figure  243.  For  the  most  part,  however,  the  chorionic  ecto- 
derm has  been  considerably  modified  from  its  primitive  condition.  The  inner  or 
cellular  layer  exhibits  irregular,  thickened  patches,  which  present  every  possible 
degree  of  variation  as  to  their  size.  A  cell  patch  from  a  somewhat  younger  stage 
is  represented  in  figure  235  as  seen  with  a  low  magnification,  and  another  patch 
of  the  age  we  are  studying  is  represented  in  figure  236.  The  patches  vary  in 
appearance;  the  cells  are  more  distinct  in  the  small  patches,  less  so  in  the  large 
patches,  in  which  there  are  often  parts  more  or  less  degenerated.  The  cell-bodies 
stain  lightly;  their  nuclei  are  granular,  not  very  sharply  defined,  and  variable  in 
size  and  shape.  The  cellular  layer  is  always  sharply  defined  against  the  mesoderm. 
Toward  the  outside  the  patches  offer  varying  relations.  In  some  cases  a  part  of 


THE  PLACENTA  IN  SITU.' 


355 


a  cell  patch  may  form  the  whole  thickness  of  the  ectoderm,  as  shown  in  figure  235, 
or  the  whole  of  a  cell  patch  may  do  so.  More  commonly,  however,  the  cellular 
patch  is  covered  more  or  less  completely  by  a  special  substance,  which  is  termed 
canalized  fibrin,  and  which  is  believed  to  represent  the  original  outer  syncytial 
layer  in  a  degenerated  condition.  The  fibrin  is  a  constant,  normal,  and  very 
remarkable  constituent  of  the  placenta.  Its  formation  seems  to  begin  always  in 
the  outer  or  syncytial  layer  of  the  chorionic  ectoderm,  but  it  may  also  spread  into 


FIG.  236. — HUMAN  CHORION  OF  SEVEN  MONTHS'  PLACENTA. 
c,  Cellular  layer,    fb,  Fibrin  layer,     ep,  Remnants  of  epithelial  layer,     mes,  Mesoderm.      X  445  diams. 


the'  cellular  layer,  which  then  becomes  replaced  by  fibrin,  so  that  this  last  alone 
represents  the  ectoderm  of  the  chorion.  .  The  fibrin  layer  consists  of  a  very  refrin- 
gent  substance  permeated  by  numerous  channels  (Fig.  236,  fb).  •  The  substance  has 
a  violent  affinity  for  carmine  and  hematoxylin,  and  hence  is  always  deeply  colored 
in  sections  stained  with  either  of  these  dyes.  The  channels  tend  to  run  more  or 
less  parallel  to  the  surface  of  the  chorion,  and  are  connected  by  numerous 
short  cross-channels.  Some  of  the  channels  contain  cells  or  nuclei.  The  appear- 
ances, however,  are  very  variable;  the  fibrin  often  sends  long  outshoots  into  the 


356 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


cellular  layers.  To  summarize,  we  may  say  that  the  ectoderm  of  the  chorionic 
membrane  undergoes  patchwise  manifold  changes.  It  exists  in  three  general  forms: 
the  nucleated  protoplasm  or  syncytium,  the  cellular  layer,  and  the  canalized  fibrin. 
A  patch  of  the  ectoderm  may  consist  of  any  one  of  these  modifications  or  any  two, 
or  of  all  three.  But  they  have  fixed  relative  positions,  for  when  the  syncytium  is 
present,  it  always  covers  the  free  surface  of  the  chorion;  when  the  cellular  layer  is 
present,  it  always  lies  next  the  mesoderm;  and  when  all  three  forms  are  present 
over  the  same  part,  the  fibrin  is  always  the  middle  stratum. 

The  mesoderm  of  the  chorion  in  early  stages  has  a  homogeneous  matrix,  which 
about  the  ninth  week  begins  to  change  its  appearance.  In  the  frondosum,  in  our 
specimen,  the  matrix  has  acquired  a  distinctly  fibrous  structure.  Usually  the  pro- 
duction of  fibers  is  much  greater  in  the  immediate  neighborhood  of  the  ectoderm, 


FIG.  237. — ADENOID  TISSUE  FROM  A  VILLUS  or  A  HUMAN  PLACENTA  or  FOUR  MONTHS. 

/,  /,  /,  Degenerating   blood-cells,     v,  v,    Capillary  blood-vessels,     d,  Finer  meshwork   surrounding  a   capillary. 

X  352  diams. 


and  this  may  go  so  far  as  to  mark  out  a  more  or  less  distinct  subectodermal 
fibrillar  layer  (Fig.  235,  Fib).  There  appears  to  be  no  mesothelial  layer  upon  the 
chorion  at  this  stage,  but  it  seems  possible  that  its  presence  might  be  revealed  by 
the  application  of  proper  special  methods. 

In  the  mill  the  ectoderm  differs  from  that  of  the  chorionic  membrane  in 
several  respects:  (i)  The  cellular  layer  after  the  first  month  becomes  less  and  less 
conspicuous,  and  after  the  fourth  month  is  present  only  in  a  few  isolated  patches, 
which  have  been  termed  the  cell-knots.  (2)  For  the  most  part  the  villi  remain 
covered  by  the  syncytial  layer,  which  in  many  places  is  thickened.  In  later  stages 
these  thickenings  are  small  and  numerous,  constituting  the  so-called  proliferation 
islands  with  many  nuclei.  Many  of  the  little  thickenings  appear  in  sections  of 


DECIDUA  SEROTINA  AT  SEVEN  MONTHS.  357 

the  villi,  and  here  and  there  are  converted  into  canalized  fibrin.  (3)  The  prolifera- 
tion islands  are  converted  into  canalized  fibrin  and  at  the  same  time  grow  and 
fuse,  forming  larger  patches,  particularly  on  the  larger  stems.  In  this  manner  are 
produced  the  large  areas  and  columns  of  fibrin  such  as  appear  in  our  section.  (4) 
Over  the  tips  of  the  villi,  where  they  are  imbedded  in  the  decidua  serotina, 
the  epithelium  apparently  degenerates  and  becomes  hyaline  tissue,  but  without 
canalization.  The  mesoderm  exists  in  two  principal  forms,  adenoid  tissue  and 
fibrillar  tissue  around  the  blood-vessels.  The  adenoid  tissue  (Fig.  237)  may  be 
considered  as  the  proper  tissue  of  the  villus.  It  consists  of  a  network  of  proto- 
plasmic threads,  which  start  from  nucleated  masses.  There  are  many  large  meshes, 
which  are  partly  occupied  by  the  very  large,  coarsely  granular  cells,  /,  I,  which  gen- 
erally are  widely  scattered,  but  sometimes  are  present  in  large  numbers.  These  free 
cells  are  extravasated  blood-corpuscles,  which  have  increased  in  size.  Probably 
they  are  dead  or  at  least  dying  and  have  swollen  by  imbibition.  They  undergo 
disintegration,  their  protoplasm,  becoming  vacuolated;  the1  vacuoles  increase  in  size 
as  the  protoplasm  is  dissolved,  until  finally  the  cell-body  entirely  disappears.  About 
the  capillary  blood-vessels,  v,  the  network  is  more  finely  spun.  Around  the  larger 
blood-vessels  the  mesoderm  has  a  distinct  intercellular  substance  with  a  ten- 
dency to  fibrillar  differentiation  in  quite  a  wide  zone  around  the  blood-vessels.  In 
this  zone  the  cells  become  elongated  or  irregularly  fusiform.  Around  the  larger 
vessels  the  cells  are  grouped  in  laminae,  and  apparently  are  contractile,  so  that  they 
must  be  looked  upon  as  an  imperfectly  differentiated  form  of  smooth  muscular 
tissue. 

Decidua  Serotina  at  Seven  Months. 

Specimens  may  be  treated  as  described  for  the  placenta  in  situ  (page  352). 
If,  however,  the  best  results  are  desired,  the  whole  of  the  uterus  should  be  cut 
through  and  the  placenta  divided  into  smaller  pieces  from  i  to  2  cm.  in  diameter, 
so  as  to  allow  a  freer  penetration  of  the  preserving  fluid.  Either  Zenker's  or 
Tellyesnicky's  fluid  is  recommended.  In  a  normal  uterus  about  seven  months 
pregnant  we  find  the  following  relations :  The  serotina  is  about  i .  5  mm.  thick, 
and  contains  an  enormous  number  of  decidual  cells  (Fig.  238);  the  cavernous, 
D' ',  and  compact,  D",  layers,  are  very  clearly  separated;  the  mucosa  is  sharply 
marked  off  from  the  muscularis,  although  scattered  decidual  cells  have  penetrated 
between  the  muscular  fibers.  The  muscularis  is  about  10  mm.  thick  and  is 
characterized  by  the  presence  of  quite  large  and'  numerous  venous  thrombi,  espe- 
cially in  the  part  toward  the  decidua.  The  decidua  itself  contains  few  blood-vessels. 
Upon  the  surface  of  the  decidua  can  be  distinguished  a  special  layer  of  denser 
decidual  tissue,  which  in  many  places  is  interrupted  by  the  ends  of  the  chorionic 
villi  which  have  penetrated  it,  as  is  well  shown  in  the  accompanying  figure.  The 
gland  cavities  of  the  spongy  layer,  D' ',  are  long  and  slit-like;  they  are  filled  for 
the  most  part  with  fine  granular  matter,  which  stains  light  blue  with  hematoxylin; 


358 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


they  also  contain  a  little  blood,  and  sometimes  a  few  decidual  cells.  There  also 
occur  in  them  hyaloid  concretions — oval  bodies  several  times  larger  than  any  of 
the  decidual  cells,  and  presenting  a  vacuolated  appearance.  In  uteri  over  two 
months  pregnant  they  are  probably  invariably  present.  In  many  places  the  glan- 
dular epithelium  is  perfectly  distinct;  its  cells  vary  greatly  in  appearance,  neighbors 
being  often,  quite  dissimilar;  nearly  all  are  cuboidal,  but  some  are  flattened  out; 
of  the  former,  a  number  are  small  with  darkly  stained  nuclei,  but  the  majority 
of  the  cells  are  enlarged,  with  greatly  enlarged,  hyaline,  very  refringent  nuclei. 


VI 


D' 


© 


me 

FIG.  238. — THE  HUMAN   DECIDUA  SEROTINA  AT  SEVEN  MONTHS.    THE  SECTION  is  TAKEN  FROM  NEAR  THE 

MARGIN  OF  THE  PLACENTA. 

Vi,  Chorionic  villi;  the  intervillous  spaces  were  filled  with  maternal  blood,  which  is  not  represented  in  the  figure. 
D',  Cavernous  layer  of  the  decidua.     D",  Compact  layer  of  the  decidua.     me  Muscularis. 


There  are  also  in  many  of  the  gland  spaces  isolated  enlarged  cells  which  have 
detached  themselves  from  the  wall,  and  in  some  cases  the  detached  cells  nearly 
fill  the  gland  cavity,  very  much  as  in  figure  230. 

The  decidual  cells  of  the  cavernous  layer  (Fig.  238,  D'}  are  smaller  and  more 
crowded  than  most  of  those  of  the  compact  layer.  The  largest  cells  are  scattered 
through  the  compact  layer,  but  are  most  numerous  toward  the  surface.  They 
extend  around  the  margin  of  the  placenta  and  have  penetrated  the  chorion,  in 
the  cellular  layer  of  which  they  are  very  numerous;  the  immigration  imparts  to 
the  chorionic  layer  in  question  somewhat  the  appearance  of  a  decidual  membrane. 
Misled  by  this  peculiarity,  some  authors  have  held  this  layer  to  be  maternal  in 


THE  HUM  A  N.  PLA  CEN  TA . 


.3.-)!) 


origin,    and    accordingly    have    described    it   as    a    "decidua   subchorialis"      The    de- 

cidual    cells    exhibit    great    variety    in    their   features    (Fig.    239).     They    are    nearly 

all  oval  discs,  so  that  their  outlines  differ  according  as  they  are  seen  lying  in  the 

tissue   turned   one   way   or  another;   they   vary   greatly  in   size;   the   larger  they   are, 

the^more  nuclei  they  contain;  the  nuclei 

are    usually    more   or  less   elongated;   the 

contents   of   the   cell   granular.      Some   of 

the   cells    present   another    type,    c;    these 

are    more    nearly    round,    are     clear    and 

transparent;    the   nucleus  is  round,  stains 

lightly,    and    contains    relatively   few   and 

small   chromatin   granules;   such   cells  are 

most  numerous  about  the  placental  margin. 


The  Human  Placenta. 

Specimens  of  the  fresh  normal  human 
placenta  may  be  obtained  without  diffi- 
culty from  maternity  hospitals.  The 
placenta  should  be  thoroughly  examined 


FIG.     239. — DECIDUAL    CELLS    FROM    THE    SECTION 

REPRESENTED  IN  FIGURE  238. 
in     the     fresh     State     by     the     Student     and      c>  Multinucleate  cell;  at  a  seven  blood-corpuscles  ha\*e 

all    the    points    in    the    description  below 

verified  by  him.  To  make  an  injected  specimen  either  the  starch  injection  mass 
or  the  colored  gelatin  mass  may  be  used  according  as*it  is  desired  to  demonstrate 
only  the  coarser  or  all  the  branches  of  the  vessels.  The"  injection  should  be  made 
through  one  of  the  arteries  of  the  umbilical  cord.  As  there  is  almost  invariably 
a  cross-anastomosis  between  the  two  arteries  close  to  the  placenta,  it  is  sufficient 
to  inject  one  of  them  in  order  to  fill  the  entire  system  of  vessels.  The  starch  mass 
may  be  injected  in  the  cold  specimen.  If  the  gelatin  mass  is  used,  the  specimen 
must  be  submerged  in  warm  water  until  it  is  sufficiently  heated  to  keep  the  gelatin 
mass  melted  during  the  process  of  injection.  After  the  gelatin  injection  is  com- 
pleted, the  placenta  may  be  preserved  in  70  per  cent  alcohol,  to  every  TOO  c.c.  of 
which  2  c.c.  of  hydrochloric  acid  have  been  added.  After  twenty-four  hours  replace 
the  acidulated  alcohol  by  fresh  alcohol  of  70  per  cent,  which  should  be  again 
changed  after  another  twenty-four  hours.  Specimens  will  then  keep  indefinitely. 
Such  specimens  may  be  used  either  for  sections  of  the  placenta  to  be  made  from 
pieces  imbedded  in  celloidin,  or  for  the  study  of  isolated  fragments  of  the  villi, 
which  are  pulled  out  of  the  placenta  by  forceps. 

The  human  placenta  is  a  disc  of  tissue  to  which  the  umbilical  "cord  of  the 
child  is  attached  by  its  distal  end.  As  a  result  of  normal  labor  the  amnion 
and  chorion,  by  which  the  fetus  in  utero  is  surrounded,  are  ruptured;  the  child 
is  then  expelled,  but  by  means  of  the  long  umbilical  cord  remains  attached  to  the 
uterus;  after  an  interval  the  placenta,  with  which  the  cord  retains  its  connection, 


360  HUMAN  UTERUS  AND  FETAL  APPENDAGES. 

is  loosened  from  the  uterine  wall  and  expelled,  together  with  the  fetal  envelopes 
and  portions  of  the  decidual  membranes  (uterine  mucosa)  of  the  mother;  the  parts 
thus  thrown  off  secondarily  constitute  the  so-called  after-birth  of  obstetricians. 

The  placenta  at  full  term,  as  thus  obtained  by  natural  expulsion,  is  a  moist 
mass,  containing  a  great  deal  of  blood,  spongy  in  texture,  about  7  inches  in 
diameter,  but  very  variable  in  size,  being  roughly  proportionate  to  the  bulk  of 
the  child;  usually  oval,  sometimes  round,  but  not  infrequently  irregular  in  shape. 
One  surface  is  smooth  and  covered  by  a  pellucid  membrane  (the  amnion),  and 
reddish  gray  in  color;  to  this  surface  the  umbilical  cord  is  attached,  and  it  shows 
the  arteries  and  veins  branching  out  irregularly  from  the  cord  over  the  surface  of 
the  placenta  (Fig.  240).  The  opposite  surface  is  rough,  lacerated,  and  usually 
covered  irregularly  with  more  or  less  blood,  which  is  often  dark  and  clotted. 
When  the  blood  is  removed,  the  surface  is  seen  to  be  crossed  by  a  system  of 
grooves  which  divide  the  placental  tissue  into  irregular  areas,  each  perhaps  an  inch 
or  so  in  diameter;  these  areas  are  called  cotyledons.  The  placenta  is  about  25  or 
30  mm.  thick,  but  thins  out  rapidly  at  the  edges,  and  its  tissue  passes  over  from 
the  margin  of  the  placenta. 

When  in  situ,  the  placenta  is  fastened  to  the  walls  of  the  uferus  by  its  rough 
or  cotyledonary  surface;  its  smooth,  amniotic  surface  faces  the  cavity  in  which  the 
fetus  lies. 

A  more  detailed  examination  of  the  gross  appearance  of  a  placenta  discharged 
at  term  leads  to  the  following  additional  observations:  The  color  is  a  reddish  or 
purplish  gray,  varying  in  tint  according  to  the  condition  of  the  blood,  and  mottled 
between  the  divaricating  blood-vessels  by  patches  and  networks  of  pale  yellowish 
or  flesh  color.  The  light  pattern  is  produced  by  the  tissue  of  the  villi  shining 
through  the  membrane  of  the  chorion.  These  appearances  are  less  distinct  when 
the  placenta,  as  is  usually  the  case,  is  covered  by  the  thin  amnion.  The  amnion, 
however,  is  very  easily  detached  as  far  as  the  insertion  of  the  umbilical  cord,  to 
the  end  of  which  it  is  firmly  attached,  but  it  cannot  be  traced  farther  because  on 
the  cord  itself  there  is  no  amnion.  The  blood-vessels  run  out  in  all  directions 
from  the  end  of  the  cord;  each  vessel  produces  a  ridge  upon  the  placental  surface, 
so  that  its  course  is  readily  followed.  The  arteries  and  veins  are  more  easily 
distinguished  after  double  injection,  as  is  shown  in  figure  240. 

The  two  kinds  of  vessels  do  not  run  together;  the  arteries  lie  near  the  surface, 
just  above  the  veins;  the  arteries  fork  repeatedly,  until  they  are  represented  only 
by  small  branches  and  fine  vessels;  some  of  the  small  branches  disappear  quite 
suddenly  by  dipping  down  into  the  deeper-lying  tissue  in  order  to  pass  into  the 
villi.  The  veins  (Fig.  240)  are  considerably  larger  than  the  arteries;  they  branch  in 
a  similar  manner,  but  some  of  the  trunks  disappear  from  the  surface  more  abruptly 
than  is  the  case  with  the  arteries.  There  is  the  greatest  possible  variability  in 
the  vessels  of  the  placenta;  one  never  sees  two  placentae  with  vessels  alike. 

The    insertion    of    the  cord    is    always    eccentric;    the    degree  of    eccentricity  is 


THE  HUMAN  PLACENTA.  361 

variable  and  is  easily  seen  to  be  related  to  the  distribution  of  the  vessels.  The 
insertion  may  even  be  entirely  outside  the  placenta,  which  yet  may  otherwise 
be  normally  developed.  Such  insertions  are  called  velamentous.  The  usual 
type  is  shown  in  figure  240.  The  arteries  come  down  together  from  the  cord; 
they  usually,  but  not  always,  anastomose  by  a  short  transverse  vessel,  which 
lies  about  half  an  inch  above  the  surface  of  the  placenta;  it  could  not  be  shown 


FIG.  240.— HUMAN  PLACENTA  AT  FULL  TERM,  DOUBLY  INJECTED  TO  SHOW  THE  SUPERFICIAL  DISTRIBUTION  OF 

THE  BLOOD-VESSELS. 
The  veins  are  drawn  dark  and  lie  deeper  than  the  arteries.     One  half  natural  size. 

in  the  figure.  Very  rarely,  if  ever,  are  there  any  arterial  or  venous  anastomoses 
on  the  surface  of  the  placenta.  The  arteries  there  spread  out  in  a  manner  which 
may  be  described  as  roughly  symmetrical.  The  veins  partially  follow  the  course 
of  the  arteries.  When  the  cord  is  inserted  near  the  margin  the  symmetry  of  the 
placental  vessels  is  greater,  when  the  insertion  is  near  the  center  the  symmetry  is 
less,  than  in  the  figure. 


362 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


The  reverse  or  uterine  surface  of  the  placenta  is  rough  and  divided  into 
numerous  rounded,  oval,  or  angular  portions  termed  lobes  or  cotyledons,  as  stated 
above.  These  vary  from  half  an  inch  to  an  inch  and  a  half  in  diameter.  The 
whole  of  this  surface  consists  of  a  thin,  soft,  somewhat  leathery  investment  by 
the  decidual  membrane,  which  dips  down  in  various  parts  to  form  the  grooves  that 
separate  the  cotyledons  from  each  other.  This  layer  is  a  portion  of  the  decidua 
serotina,  which,  as  long  as  the  parts  are  in  situ,  constitutes  the  boundary  between 


si 


FIG.  241. — HUMAN  PLACENTA  AFTER  DELIVERY  AT  FULL  TERM. 

A,  Vertical  section  through  the  margin:  D,  decidua;  vi,  aborted  villi  outside  the  placenta;  Cho,  chorion;5&,  sinus; 
Vi,  placental  villi;  Fib,  fibrin.  B,  Portion  of  A  more  highly  magnified  to  show  the  decidual  tissue  near  b: 
v,  blood-vessel;  d,  decidual  cell  with  one  nucleus;  d',  decidual  cell  with  several  nuclei. 

the  placenta  and  the  muscular  substance  of  the  uterus,  but  which  at  the  time  of 
labor  becomes  split  asunder,  so  that,  while  a  portion  is  carried  off  along  with  the 
placenta  and  constitutes  its  external  membrane,  the  rest  remains  attached  to  the 
inner  surface  of  the  uterus.  If  a  placenta  is  cut  through,  if  is  found  to  consist  of 
a  spongy  mass  containing  a  large  quantity  of  blood  and  bounded  by  two  mem- 
branes, each  less  than  a  millimeter  thick;  the  upper  one  is  the  chorion,  covered 
by  the  still  thinner  amnion,  and  greatly  thickened  where  the  vessels  lie  in  it;  the 
lower  one  is  the  decidual  tissue,  together  with  the  ends  of  the  villi  imbedded  in 


HISTOLOGY  OF  THE  HUMAN  CH ORION.  363 

it  (cf.  especially  page  357  and  Fig.  238);  it  represents  only  a  portion  of  the  de- 
cidua,  the  other  portion  having  remained  upon  the  uterine  wall.  The  spongy  mass 
is  found  upon  examination  to  consist  of  an  immense  number  of  tufts  of  fine  rods 
of  tissue,  which  are  irregularly  cylindrical  in  shape.  Further  examination  shows 
that  they  are  twigs  (Fig.  248)  with  rounded  ends  and  springing  from  branchlets 
which  in  their  turn  arise  from  branches,  and  so  on  until  a  large  main  stem  is 
found,  which  starts  from  the  chorion.  This  branching  system  is  richly  supplied  with 
blood  from  the  fetal  vessels  on  the  surface  of  the  placenta.  The  villi  are  inter- 
woven so  that  the  twigs  of  one  branch  are  interlaced  with  those  of  another,  and 
apparently  separate  twigs  may  grow  together  and  their  vessels  anastomose;  but 
on  this  point  we  are  unable  to  speak  positively.  The  villous  twigs  next  the  surface 
of  the  decidua  penetrate  that  tissue  a  slight  distance. 

The  intervillous  spaces  are  filled,  or  nearly  so,  with  blood;  they  form  a  com- 
plex system  of  channels.  The  intervillous  blood  is  maternal.  Farre  says,  in  his 
article  in  Todd's  "Cyclopaedia"  (Vol.  V,  page  716),  in  reference  to  the  placental 
decidua:  "Numerous  valve-like  apertures  are  observed  upon  all  parts  of  the  surface. 
They  are  the  orifices  of  the  veins  which  have  been  torn  off  from  the  uterus.  A 
probe  passed  into  any  one  of  these,  after  taking  an  oblique  direction,  enters  at 
once  into  the  placental  substance.  Small  arteries,  about  half  an  inch  in  length,  are 
also  everywhere  observed  embedded  in  this  layer.  After  making  several  sharp 
spiral  turns,  they  likewise  suddenly  open  into  the  placenta";  and  on  page  719  he 
adds:  "These  venous  orifices  occupy  three  situations.  The  first  and  most  numerous 
are  scattered  over  the  inner  side  of  the  general  layer  of  decidua  which  constitutes 
the  upper  boundary  of  the  placenta;  the  second  form  openings  upon  the  sides  of 
the  decidual  prolongations-- or  dissepiments,  which  separate  the  lobes  [cotyledons] 
from  each  other;  while  the  third  lead  directly  into  the  interrupted  channel  in  the 
margin,  termed  the  circular  sinus."  The  circular  sinus  (Fig.  241,  Si)  is  merely  a 
space  at  the  edge  of  the  placenta  which  is  left  comparatively  free  from  the  villi. 
It  is  not  a  continuous  channel,  but  is  interrupted  here  and  there.  Subsequent 
writers  have  gone  but  little  beyond  Farre's  account,  which  has  been  entirely  over- 
looked by  most  recent  investigators,  who,  accordingly,  have  announced  as  new 
discoveries  many  facts  known  to  Farre.  Under  these  circumstances  it  is -interesting 
to  direct  renewed  attention  to  Farre's  masterly  article. 

Histology  of  the  Human  Chorion. 

The  chorion  may  be  preserved  in  Zenker's  or  Tellyesnicky's  fluid  or  in 
Bouin's  picro-formalin  fluid.  Pieces  may  be  stained  in  toto  with  alum  cochineal 
or  borax  carmine  and  transverse  sections  cut  in  paraffin.  The  sections  may  be 
advantageously  counterstained  with  eosin  or  orange  G. 

For  the  general  history  of  the  chorion  see  page  82.  As  it  is  formed  by  the 
somatopleure,  it  comprises  an  outer  ectoderm  and  an  inner  mesoderm,  which 
latter  comprises  mesenchyma  and  mesothelium. 


364  '  HUMAN  UTERUS  AND  FETAL  APPENDAGES. 

The  ectoderm  undergoes  a  very  precocious  growth  producing  a  very  large 
number  of  cells,  which  form  the  thick  trophodermic  layer  as  described  on  page  365. 
Then  follows  the  stage  in  which,  by  degeneration,  spaces  are  produced  in  the 
trophoderm  into  which  the  blood  of  the  mother  enters  and  circulates;  and  at  the 
same  time  prolongations  of  the  chorionic  mesoderm  extend  into  the  trophoderm. 
The  ectodermal  cells  arrange  themselves  as  a  covering  for  these  mesodermic 
outgrowths  and  so  complete  a  villus.  The  trophoderm  between  the  developing 
villi  entirely  disappears.  The  ectoderm,  which  covers  both  the  villi  and  the 
chorionic  membrane  proper,  consists  of  two  layers,  an  inner  cellular  and  an  outer 
syncytial  layer.  Much  of  the  trophoderm  may  still  remain  for  awhile  around 
and  beyond  the  tips  of  the  villi,  but  it  disappears  rapidly,  probably  during  the 
third  week,  so  that  the  villi  alone  are  left.  The  two-layered  stage  of  the  ectoderm 
is  only  partially  preserved  during  the  later  development.  Many  parts  of  it  become 
thinned  out  so  as  to  contain  only  one  layer  of  cells,  while  other  parts  thicken 
and  degenerate.  These  changes  may  be  studied  in  sections  of  older  placentas 
(see  Fig.  234). 

The  mesoderm  of  the  chorion  consists  at  first  of  mesenchymal  cells  with  a 
homogeneous  matrix  and  a  layer  of  mesothelium.  In  later  stages  the  mesen- 
chymal tissue  becomes  partly  fibrillar,  and  it  is  doubtful  whether  the  mesothelium 
persists  or  not.  During  the  third  week  we  find  the  chorion  vascular.  ..  Around 
the  larger  blood-vessels  the  mesoderm  forms  a  more  or  less  distinct  coat  in  which 
the  cells  are  somewhat  more  crowded  together  in  laminae.  After  the  perivascular 
coats  have  acquired  a  certain  thickness  the  cells  of  their  inner  portions  become 
more  elongated,  more  regularly  spindle-shaped,  and  more  closely  packed  than 
those  of  the  outer  layer.  The  transition  from  the  denser  to  the  looser  tissue  is 
gradual.  We  are  perhaps  entitled  to  call  the  denser,  inner  layer  the  media,  and 
the  outer,  looser  layer  the  adventitia,  although  neither  of  the  layers  has  by  any 
means  the  full  histological  differentiation  characteristic  of  the  like-named  layers 
of  the  blood-vessels  of  the  adult.  The  histogenetic  changes  in  the  chorion 
frondosum  go  further  than  in  the  chorion  Iseve,  which  may  be  said  to  be,  as  it 
were,  arrested  in  its  development. 

The  Chorion  with  Trophoderm. 

When  the  chorionic  vesicle  has  an  internal  diameter  of  from  3  to  6  or  7  mm., 
it  will  be  found  to  exhibit  well-developed  trophodermic  layers.  Such  a  vesicle 
may  be  hardened  in  Zenker's  fluid  or,  better,  in  Flemming's  or  Hermann's  fluid, 
as  these  produce  at  the  same  time  a  differential  color  (Fig.  242).  The  chorionic 
membrane  is  quite  thin,  and  consists  chiefly  of  mesoderm,  mes,  with  a  covering 
of  ectoderm,  EC,  consisting  of  two  layers  of  cells.  The  mesoderm  extends  down 
to  form  the  core  of  the  villi  shown.  These  villi  are  much  branched  and  are  also 
covered  by  a  layer  of  ectoderm.  At  the  denser  ends  of  the  villi  the  ectoderm  is 
very  much  thickened,  forming  a  great  mass  of  cells,  so  that  the  ectoderm  con- 


THE  CH ORION  WITH  TROPHODERM. 

nected  with  one  villus  is  fused  with  that  of  adjacent  villi,  the  whole  constituting 
a  large  irregular  mass  of  cells,  Tro,  the  trophoderm.  In  many  places  it  has 
already  disappeared,  so  that  there  are  spaces,  lac,  in  the  trophoblastic  mass.  On 
the  edges  of  these  spaces  the  trophoblast  is  undergoing  degeneration,  deg,  and 
where  that  is  occurring  it  is  marked  in  the  figure  by  the  deeper  staining  of  the 
degenerated  material.  Upon  examination  with  a  higher  power  (Fig.  243)  it  will 
be  noted  that  the  mesodermic  cells  are  stained  much  more  deeply  than  the  matrix. 


ntes. 


EC. 


lac. 


Tro. 


FIG.  242.— SECTION  OF  A  VERY  YOUNG  HUMAN  CHORION. 

deg,  Degenerating  ectoderm.     EC,  Epithelial  ectoderm,     lac,  Lacuna  for  maternal  blood,     mes,  Mesoderm.     Tro, 

Trophoderm.     Vi,  Villi.-     X  50  diams. 


They  hav.e  an  elongated  form  and  run  in  various  directions,  more  or  less  parallel 
to  the  epithelium,  EC'.  Many  of  them  are  cut  transversely  or  obliquely.  Aside 
from  the  trophoderm,  the  ectoderm  is  everywhere  two-layered.  The  inner  layer 
is  distinctly  cellular,  the  outlines  of  the  cells  being  very  sharply  marked  and  the 
nuclei  being  relatively  large.  In  the.  outer  layer,  which  is  stained  more  darkly, 
there  are  no  cell  boundaries  to  be  recognized,  the  structure  being  syncytial.  The 
nuclei  are  smaller  and  more  deeply  stained  than  those  of  the  inner  layer.  •  In 


366 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


the  trophoderm  we  find  great  masses  of  cells  somewhat  similar  to  those  of  the 
cellular  layer  upon  the  chorionic  membrane  and  over  the  surface  of  the  villi,  but 
they  are  larger  and  more  lightly  stained.  They  lie  closely  packed  together;  their 
nuclei  are  rounded  in  form,  but  vary  considerably  in  size  and  shape.  Many  of 
them  contain  one  or  two  distinct  spots,  which,  however,  are  sometimes  absent. 
On  the  edges  of  the  spaces  which  have  been  formed,  and  sometimes  apparently 
in  the  interior  of  the  mass  of  trophoderm,  we  find  bands  and  lines  of  degenerative 


EC'. 


EC". 


Tro. 


FIG.  243. — PORTION  OF  THE  PRECEDING  FIGURE  MORE  HIGHLY  MAGNIFIED. 

deg,  Degenerating  ectoderm.     EC',  Outer  syncytial  layer  of  ectoderm.     EC",  Inner  cellular  layer  of  ectoderm. 
mes,  Mesoderm  of  villus.     Tro,  Trophoblast.      X  350  diams. 


material  in  which  we  can  find  nuclei,  but  no  distinct  cell  boundaries.  The 
substance  between  the  nuclei  is  more  or  less  uniformly  granular  in  texture  and 
stains  quite  deeply.  The  nuclei  of  the  degenerative  material  vary  extremely  in 
appearance.  In  some  cases  they  are  small  and  stain  rather  deeply,  and  are  then 
found  to  be  present  in  more  or  less  considerable  numbers.  Occasionally,  however, 
the  nuclei  are  much  larger,  and  more  rarely  one  sees  a  nucleus  of  exceptionally 
great  diameter. 


THE  CHORIONIC  VILLI.  367 

Our  knowledge  of  the  human  trophoderm  being  still  very  imperfect,  its  full 
history  is  partly  a  matter  of  supposition.  The  appearances  described  indicate 
that  the  trophoderm  undergoes  a  rapid  degeneration,  during  which  the  cells  fuse, 
while  their  protoplasm  becomes  a  hyaline  material.  -  We  must  then  further  suppose 
that  •  the  degenerated  substance  is  resorbed  and  disappears  altogether.  Finally, 
we  must  assume  that  the  entire  trophoderm  does  not  disappear,  but  that  enough 
is  preserved  to  form  the  permanent  covering  of  the  villi. 

It  may  be  noted  that  the  specimen  on  which  the  above  description  is  based 
agrees  essentially  with  the  specimen  described  by  Siegenbeek  van  Heukelom,  which 
is  regarded  as  normal. 

The  Chorionic  Villi. 

The  villi  may  be  obtained  in  connebtion  with  the  preparations  of  the  uterus 
and  placenta.  In  order  to  see  the  youngest  stages  of  the  first  villi  it  "is  necessary 
to  have  the  chorionic  membrane  of  the  second  or  early  part  of  the  third  week.  At 
this  stage  the  trophoderm  is  present  and  the  first  villi  are  appearing  (compare  page 
115).  To  study  the  growth  and  form  of  the  villi,  single  villi  or  pieces  of  villi 
should  be  snipped  off  from  the  chorion  at  various  stages.  Such  pieces  may.  be 
examined  as  opaque  objects  in  alcohol,  or  they  may  be  •  stained  and  mounted  as 
permanent  preparations.  To  obtain  injected  villi  it  is  best  to  inject  the  placenta 
through  one  of  the  arteries  of  the  umbilical  cord,  using  as  the  injecting  mass 
gelatin  colored  with  carmine  or  Prussian  blue.  Such  injections  are  very  easily 
made. 

Branching  of  the  Villi. — The  formation  of  a  branch  is  usually  initiated  by  an- 
outgrowth  of  the  ectoderm.  Branches  grow  very  rapidly;  the  outgrowth  which 
forms  the  branch  occurs  with  every  degree  of  participation  of  the  mesoderm. 
The  two  extremes  are,  first,  the  bud  consisting  wholly  of  epithelium,  which  may 
become  a  process  with  a  long,  thin  pedicle  and  a  thickened  free  end  remaining 
sometimes  entirely  without  mesoderm;  later  the  mesoderm  penetrates  it  and 
completes  the  structure.  Second,  a  thick  bud  with  a  well-developed  cord  of  con- 
nective tis'sue  and  having  a  nearly  cylindrical  form.  Between  these  extremes  every 
intermediate  stage  can  be  found.  The  tips  of  the  branches  are  for  the  most  part 
free,  but  some  of  them  come  in  contact  with  the  surfaces  of  the  decidua  and 
penetrate  it  for  a  short  distance.  By  this  means  the  villi  of  the  embryo  are 
attached  to  the  decidua  of  the  mother.  The  villi  do  not  penetrate  the  glands 
of  the  uterus  at  any  period,  as  was  at  one  time  supposed.  The  ectoderm  on  the 
tip  of  the  villi,  where  it  is  in  contact  with  decidual  tissue,  undergoes  a  hyaline 
degeneration. 

The  shape  of  the  villi  varies  according  to  the  part  of  the  chorion  and  the  age 
of  the  embryo.  Over  the  chorion  laeve  there  is  first  an  arrest  of  development  and 
a  subsequent  slow  degeneration  of  the  tissues  which  lose  all  recognizable  organ- 
ization of  the  protoplasm,  and  to  a  large  extent  of  their  nuclei  also.  At  the  same 


368  HUMAN  UTERUS  AND  FETAL  APPENDAGES. 

time  the  villi  alter  in  shape  (Fig.  244),  becoming  more  and  more  filamentous.  By 
the  fourth  month  only  a  few  tapering  threads  with  very  few  branches  remain. 
The  villi  disappear  almost  completely  from  the  chorion  laeve,  except  near  the  edge 
of  the  placenta.  The  villi  of  the  chorion  frondosum  or  placental  region,  on  the 
contrary,  make  an  enormous  growth.  At  first  they  are  short,  thick-set  bodies  of 
irregular  shape,  as  shown  in  figure  245.  At  twelve  weeks  their  form  is  ex- 
tremely characteristic  (Figc  246).  The  main  stem  gives  off  numerous  branches 
at  more  or  less  acute  angles,  and  these  again  other  branches,  until  at  last  the 
terminal  twigs  are  reached.  The  branches  are  extremely  irregular  and  variable, 
though  in  general  club-shaped  and  constricted  at  the  base.  The  branches  may 
be  bigger  than  the  trunk  which  bears  them,  or  of  any  less  size.  In  older  stages 


FIG.   244. — ABORTING  VILLUS   FROM  THE  HUMAN  FIG.  245. — FRAGMENT  OF  THE  CHORION  OF  FIGURE 

CHORION  L^VE  OF  THE  SECOND  MONTH.  84,  HIGHLY  MAGNIFIED. 

EC,  Ectoderm.     Mes,  Mesoderm.     Vi,  Villus  formed 
wholly  by  ectoderm. 

there  is  a  progressive  change.  During  the  fifth  month  we  find  the  irregularity 
of  shape,  though  still  very  marked,  decidedly  less  exaggerated  (Fig.  247).  The 
branches  tend  to  come  off  at  more  nearly  right  angles.  One  finds  very  numerous 
free  ends,  as  of  course  only  a  small  portion  of  the  branches  touch  the  decidual 
surface.  The  branches,  too,  are  less  out  of  proportion  to  the  stems,  less  constricted 
at  their  bases,  -less  awkward  in  form.  The  gradual  changes  continue  until  at  full 
term,  as  shown  by  figure  248,  the  branches  are  long,  slender,  and  less  closely 
set  as  well  as  less  subdivided  than  at  early  stages.  They  have  nodular  projec- 
tions like  branches  arrested  at  the  beginning  of  their  development.  There  are 
numerous  spots  upon  the  surfaces  of  the  villi.  Microscopic  examination  shows  that 
these  spots  are  proliferation  islands,  as  we  may  call  them,  or  little  thickenings 
of  the  ectoderm  with  crowded  nuclei.  Not  all  the  villi,  ^however,  have  changed  to 
the  slender  form,  for  some  still  preserve  the  earlier,  clumsier  shapes.  In  sections 


THE  CHORIONIC  VILLI. 


369 


X  19 


FIG.    2^6.— ISOLATED    TERMINAL    BRANCH    OF    A 
VlLLTJS  FROM  A  HUMAN  CHORION  OF  TWELVE 

WEEKS. 


FIG.    247. — VILLOUS   STEM  FROM  A  HUMAN  PLA- 
CENTA OF  THE  FIFTH  MONTH.     X  9  diams. 


Xl9 


FIG.  248. — TERMINAL  BRANCHES  OF  A  VILLUS  FROM  A  FIG.     249. — PORTION    OF    AN    INJECTED    VILLXJS 

HUMAN  PLACENTA  AT  FULL  TERM.  FROM  A  PLACENTA  OF  ABOUT  FIVE  MONTHS. 

The    little    spots   indicate   proliferation   islands   of    the  X  210  diams. 
covering  epithelium. 


24 


370 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


of  placentas  of  different  ages  the  villi  offer  characteristic  differences;  for  the  younger 
the  stage,  the  fewer  the  total  number  of  branches  and  the  larger  their  average 
size.  The  older  the  placenta,  the  more  numerous  and  smaller  are  the  branches  as 
they  appear  in  sections  (Fig.  234). 

Injected  Villi. — The  arteries  and  veins  of  the  chorionic  membrane  enter  the 
villi.  After  a  short  course  in  the  main  stalk  of  a  villus,  the  vessels  give  rise  to 
many  branchlets,  and  gradually  the  character  of  the  circulation  changes,  until  in 

the  smallest  villous  twigs  there  are  capillaries  only  (Fig. 
249).  The  capillaries  are  remarkable  for  their  large  size, 
and  on  this  account  have  been  interpreted  as  arteries  and 
veins  by  some  of  the  older  writers.  Their  caliber  is 
often  sufficient  for  the  passage  of  from  two  to  six 
blood-corpuscles  abreast.  They  are  very  variable  in 
diameter,  and  also  peculiar,  in  exhibiting  sudden  con- 
strictions and  dilatations.  In  the  short  knob-like  branches 
there  is  often  only  a  single  capillary  loop,  but  as  the 
branch  becomes  larger  the  number  of  loops  increases 
and  they  form  anastomoses.  In  the  branches  large  enough 
to  admit  of  it,  there  are  two  (or  sometimes  only  one) 
longitudinal  central  vessels,  the  artery  and  vein  of  a 
superficial  network  of  capillaries  (Fig.  250).  The  forma- 
tion of  loops  and  the  large  size  of  the  capillaries  are  not 
especially  characteristic  of  the  villi,  but  of  the  fetal  blood- 
vessels in  general. 

The    histology   of   the    villi  is  described   in   the  section 
on   the  placenta  in  situ,   page  356. 


FIG.  250. — PORTION  OF  A 
SMALL  INJECTED  VILLOUS 
STEM  FROM  A  PLACENTA  OF 


The  Structure  of  the  Amnion. 

The   structure   of   the  amnion  may  be  studied  in  sec- 


ABOTJT  FIVE  MONTHS,  x  tions,  such  as  will  be  obtained  by  the  student  in  con- 
nection with  the  sections  of  the  chicken  and  pig  em- 
bryos. These  preparations  will  show  the  early  stages.  When  the  amnion  is 
first  formed,  it  consists  of  two  layers  of  cells,  both  very  thin,  and  with  somewhat 
widely  separated  nuclei  in  each  layer.  Sometimes  the  nuclei  lie  in  small  groups. 
Between  the  two  layers  is  a  distinct  space.  The  layer  facing  the  embryo  is  a 
continuation  of  the  embryonic  ectoderm,  and  is  more  regular  and  better  defined 
than  the  second  or  mesodermal  layer,  which  is  less  regular  and  sends  at  in- 
tervals protoplasmic  processes  across  the  space  between  the  two  layers  to  attach 
themselves  to  the  ectoderm. 

Human  Amnion  at  Two  Months. — A  section  is  shown  in  figure  251.  The 
ectoderm,  EC,  is  still  very  thin,  but  where  the  nuclei  are  placed  the  layer  is  a  little 
thicker.  The.  mesoderm,  on  the  other  hand,  has  become  quite  thick,  and  is 


THE  STRUCTURE  OF  THE  AM N ION. 


371 


readily  seen  to  be  separated  into  two  parts,  a  thin  mesothelial  layer,  Mstk,  cover- 
ing the  surface  of  the  amnion  toward  the  chorion,  and  a  mesenchymal  layer,  Mes, 
which   makes   up   the  greater  part  of  the  membrane.      Traces  of  fibrillar  structure 
in   trie   mesenchyma  are  observable.      No  blood-vessels,   lymphatics,   or  nerves   have" 
been   found. 


EC 


-:,  Mes 


Msth 


FIG.  251. — TRANSVERSE  SECTION  OF  A  HUMAN  AMNION  OF  Two  MONTHS. 
EC,  Ectoderm.     Mes,  Mesenchymal  mesoderm.     Msth,  Mesothelium.      X  250  diams. 

Human  Amnion  after  the  Fifth  Month. — This  should  be  studied  both  in  sections 
and  in  surface  views  of  the  whole  membrane,  small  pieces  being  mounted  with 
the  ectodermal  side  up.  The  preparation  may  be  stained  with  alum  .hematoxylin 
and  eosin.  Sections  show  that  the  ectoderm  (Fig.  252,  ec(]  has  grown  somewhat 


FIG.  252. — Two  SECTIONS  OF  THE  HUMAN  AMNION.  FIG.  253: — SURFACE  VIEW  OF  THE  HUMAN  AMNIOTIC 
A,  From  an  embryo  of  eight  months;  B,  at  term.  EPITHELIUM  OF  THE  FOURTH  MONTH. 

ect,    Ectoderm,     mes,    Mesoderm.     a,    Meso-  pi,    Protoplasm,     pr,    Intercellular    processes,     nu, 
thelium.      X  340  diams.  Nucleus.      X  1225  diams. 

in  thickness.  Usually  the  cells  are  cuboidal  (Fig.  252,  A),  each  with  a  rounded 
top  in  which  is  situated  the  more  or  less  nearly  spherical  nucleus.  Sometimes, 
however,  the  nuclei  lie  deeper  down.  Less  frequently  the  epithelium  is  thin 
(Fig.  252,  B),  and  its  nuclei,  which  are  transversely  elongated,  lie  farther  apart. 
As  regards  the  mesoderm,  it  will  be  noticed  that  there  is  usually,  perhaps  always, 


372 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


a  layer  of  nearly  homogeneous  basal  substance  or  matrix  immediately  underneath 
the  ectoderm  and  remarkable  for  containing  no  cells.  Sometimes  the  remaining 
portion  of  the  mesoderm  is  broken  up  so  as  to  offer  a  fibrillar  structure  (Fig.  252, 
A),  and  when  that  is  the  case  we  can  no  longer  make  out  a  distinct  mesothelial 
layer.  At  other  times  the  more  or  less  homogeneous  matrix  can  be  seen  through 
the  whole  thickness  of  the  amnion  (Fig.  252,  B),  and  when  this  is  the  case  the 
mesothelium,  a,  can  be  readily  identified. 

In  surface  views  the  amniotic  ectoderm  is  seen  to  consist  of  more  or  less 
regularly  distributed  nuclei  with  cell-bodies  connecting  with  one  another  by  inter- 
cellular bridges  of  protoplasm  (Fig.  253).  The  nuclei,  nu,  are  relatively  large, 

rounded,  and  with  distinct  outlines.  They 
have  a  more  or  less  well-marked  intranuclear 
network  with  thickened  nodes  and  a  small 
number  of  deeply  stained  granules  which 
are  probably  chromatin.  Each  nucleus  is 
surrounded  by  a  cell-body,  pi,  and  the  ad- 
jacent cell-bodies  are  separated  from  one 
another  by  clear  spaces  which  are  crossed 
by  threads  of  material,  pr,  stretching  as 
bridges  between  the  neighboring  cells.  The 
protoplasm  is  vacuolated.  The  whole  picture 
thus  leads  to  the  view  that  the  epithelium 
is  a  sponge-work  of  protoplasm  somewhat 
condensed  around  each  nucleus.  As  re- 
gards the  mesoderm,  it  is  very  difficult  to 
obtain  clear  pictures  of  the  cells,  though 
the  nuclei  can  be  readily  observed.  They  vary  greatly  in  appearance,  being  some- 
times fairly  regular  and  uniform,  though  always  far  less  so  than  the  nuclei  of  the 
mesenchyma  of  the  embryo  proper.  In  other  cases  (Fig.  254)  the  nuclei  are  exceed- 
ingly irregular;  some  are  large  with  a  distinct  network,  d;  others  are  smaller  and 
differ  but  slightly  from  the  normal.  Some  are  very  irregular,  b,  others  slightly 
irregular,  c,  and  others  again  strangely  elongated  and  narrow,  a.  Many  other 
forms  besides  those  represented  in  figure  254  may  be  found.  It  has  been  sug- 
gested that  these  varied  shapes  of  the  nuclei  indicate  degenerative  changes,  and,  in 
fact,  many  of  the  nuclei  are  actually  breaking  down,  for  in  some  specimens  every 
stage  between  a  nucleus  and  scattered  granules  can  be  observed,  and  one  may  find 
nuclei  with  distinct  membranes,  without  membranes,  masses  of  granular  matter  stained, 
clusters  of  granules  crowded  together,  and,  finally,  other  clusters  more  or  less  scattered. 

The  Umbilical  Cord. 

The  umbilical  cord  may  be  well  preserved  in  Zenker's  or  Tellyesnicky's  fluid. 
Transverse  sections  may  be  prepared  in  paraffin  and  stained  with  alum  hematoxylin 


FIG.  254. — NATURAL  GROUP  or  NUCLEI  FROM  THE 
MESODERM  OF  THE  HUMAN  AMNION  OF  THE 
FIFTH  MONTH.  (For  lettering  see  text.) 
X  1225  diams. 


THE  UMBILICAL  CORD. 


373 


and  eosin,  or  with  Heidenhain's  iron  hehiatoxylin  and  orange  G;  or,  if  it  is 
desired  to  study  the  development  of  the  connective-tissue  fibrillae,  with  Mallory's 
triple,  connective-tissue  stain. 


FIG.  255. — CROSS-SECTION  OF  A  HUMAN  UMBILICAL 
CORD  AT  TERM. 

.4,  A',  Umbilical  arteries  much  contracted.  V,  Um- 
bilical vein.  Y,  Remnant  of  allantois.  X  12 
diams. 


FIG.    256. — SECTION   OF   THE   ALLANTOIS   FROM  A 

HUMAN  UMBILICAL  CORD  OF  THREE  MONTHS. 

ent,  Allantoic  entoderm.     mes,  Mesoderm.     X  340 

diams. 


FJG.   ^.—CONNECTIVE  TISSUE  FROM  THE  UMBILICAL  CORD  OF  A  HUMAN  EMBRYO  OF  21  MM.      STAINED  WITH 

ALUM  COCHINEAL  AND  EOSIN. 
n,  Nucleus,    p,  Protoplasmic  network.     X  540  diams. 

A  general  description  of  the  umbilical  cord  has  been  given,  pages  115  to  116, 
and  two  sections  (Fig.  66)  are  there  represented  showing  the  structures  which 
appear  in  sections  of  the  umbilical  cord.  At  full  term  some  of  these  structures 


374 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


are  still  present  but  somewhat*  modified  (Fig.  255),  while  others  have  been  partly 
or  wholly  obliterated.  As  contrasted  with  the  early  stages,  we  find  that  the 
ccelom  is  entirely  obliterated,  that  the  yolk-stalk  has  usually  been  completely 
resorbed,  and  that  only  traces  of  the  allantois  can  be  seen,  Y.  The  blood-vessels 
have  grown;  there  are  two  arteries,  A,  A',  and  a  single  vein,  V.  Around  each  of 
these  is  a  well-developed  muscular  coat  produced  by  differentiation  of  the  sur- 
rounding mesenchymal  cells,  which  have  assumed  an  elongated  form  and  con- 


c.-,.  //-•  '.0    WVJS' 

^  -\v      ,  •      m..*"'   • 


FIG.  258. — CONNECTIVE  TISSUE  FROM  THE  UMBILICAL  CORD  OF  A  HUMAN  EMBRYO  OF  THREE  MONTHS,  STAINED 

WITH  ALUM  COCHINEAL  AND  EOSIN.     X  511  diams. 

c,c,  Cells.    /,  Fibrillae. 

tractile  function.  It  will  be  remembered  that  the  allantois  in  man  is  primitively 
a  very  narrow  tubular  diverticulum  which  extends  originally  nearly  to  the  chorion 
(compare  Fig.  87).  As  the  umbilical  cord  lengthens  the  allantois  fails  to  lengthen 
equally.  During  the  second  month  it  increases  very  little  in  diameter.  After  the 
second  month  it  appears  in  sections  as  a  small  group  of  epithelioid  cells  (Fig.  256) 
with  distinct  walls,  irregularly  granular  contents,  and  round  nuclei;  the  group  may 
or  may  not  show  a  remnant  of  the  original  central  cavity.  Around  the  cells, 
ent,  there  is  a  slight  condensation  of  the  connective  tissue,  mes,  to  form,  as  it  were, 
an  envelope. 

The    mesoderm    varies    in    appearance    according    to    the    age*  of    the    specimen. 


THE  STRUCTURE  OF  THE  HUMAN  YOLK-SAC. 

Its  growth  and  differentiation  are  rapid.  During  the  second  month  it  consists 
merely  of  numerous  cells  (Fig.  257)  imbedded  in  a  clear  substance.  The  cells 
form  a  complex  network  of  which  the  filaments  and  meshes  are  extremely  variable 
in  size.  The  nuclei  are  oval,  granular,  and  do  not  always  have  accumulations 
of  protoplasm  about  them  forming  main  cell-bodies.  (Compare  description  of 
first  stage  of  the  mesenchyma,  page  89.)  By  the  end  of  the  third  month  the  cells 
have  assumed  nearly  their  definite  form  (Fig.  258).  Their  protoplasm  is  increased 
in  amount  and  forms  a  large  body  around  each  nucleus.  The  network  has  become 
simpler  and  coarser,  the  meshes  bigger,  and  the  filaments  fewer  and  thicker.  In 
the  matrix  are  numerous  connective-tissue  fibrillae,  not  yet  disposed  in  bundles. 


FIG.  259. — ECTODERM  OF  AN  UMBILICAL  CORD  OF  A  HUMAN  EMBRYO  OF  THREE  MONTHS. 

EC,  Ectoderm,     mes,  Mesoderm.     c,  Mesenchymal  cell,    a,  Outer  layer  of  ectoderm,    b,  'Inner  layer  of  ectoderm. 

X  545  diams. 

In  older  cords  there  is  an  obvious  increase  in  the  number  of  fibrillae  and  they 
form  wavy  bundles.  In  the  cord  of  yet  older  stages  the  matrix  also  contains 
mucin  which  may  be  stained  by  alum  hematoxylin.  In  such  .cords  so  stained  the 
blotch  of  color  appears  in  the  intercellular  spaces. 

The  ectoderm  is  at  first  a  single  layer  of  cells,  as  it  is  also  over  the  body  of 
the  embryo,  and  as  it  remains  permanently  over  the  amnion.  At  three  months 
we  find  the  ectoderm  to  be  two-layered,  corresponding  to  the  second  stage  of  the 
epidermis  of  the  embryo.  In  still  older  stages  there  is  slight  increase  in  the 
number  of  layers  of  the  ectoderm,  but  it  never  passes  much  beyond  the  stage  of 
the  embryonic  epidermis  at  the  fourth  month.  Figure  259  is  from  a  cord  at  three 
months.  The  outer  layer,  a,  of  ectodermal  cells  is  granular  and  stains  much 
more  darkly  than  the  inner  layer,  b,  in  which  also  cell  bundles  are  more  distinct. 

The  Structure  of  the  Human  Yolk-sac. 

The  human  yolk-sac  may  be  preserved  in  Zenker's  or  Tellyesnicky's  fluid, 
stained  in  toto  with  alum  .cochineal,  imbedded  in  paraffin,  and  cut  in  transverse 
sections.  Yolk-sacs  of  the  second  month  are  preferable  for  study. 


376 


HUMAN  UTERUS  AND  FETAL  APPENDAGES. 


The  general  history  of  the  yolk-sac  is  described  on  pages  63  and  66.  It 
becomes  a  pear-shaped  vesicle  which  in  man  attains  its  maximum  diameter  about 
the  end  of  the  fourth  week.  It  then  measures  from  7  to  n  mm.  From  its 
pointed  end  runs  the  long  stalk  by  which  it  is  connected  with  the  intestine.  In 
very  early  stages  the  stalk  is  hollow  and  its  cavity  is  lined  by  entoderm.  But 
this  condition  is  soon  obliterated,  the  stalk  becoming  solid  and  the  entoderm  dis- 
appearing. In  this  condition  we  found  the  yolk-stalk  in  an  embryo  of  21  mm. 

(Fig.  66,  A).  The  sac  itself  remains  hollow  (Fig. 
260).  It  has  a  lining  of  entodermal  cells,  En,  and 
a  thicker  layer  of  mesoderm,  mes,  containing  blood- 
vessels, v.  The  network  of  the  vessels  imparts  a 
characteristic  appearance  to  the  external  or  meso- 
dermic  surface  of  the  yolk-sac.  In  the  earliest 
stages  observed  the  entoderm  consisted  of  a  single 
layer  of  cuboidal  cells. 

Transverse  Section  of  a  Yolk-sac  of  about  Two 
Months. — The  contents  of  the  fresh  yolk-sac  are 
fluid,  but  coagulate  when  the  organ  is  hardened. 
In  the  coagulum  are  found  some  stained  bodies 

_  which    are    supposed    to    be    yolk    material.      The 

OF  A  VERY  YOUNG  HUMAN  EMBRYO.  J 

En,  Entoderm.    mes,  Mesoderm.    v,     entoderm    has    undergone    proliferation    and    thick- 
Blood-vessel.— (After  Fr.  Keibel.)        ening.     These   cells  are   more   or  less  irregular  and 

disposed    in    two    or    three    layers.      Many    of    the 

superficial  cells  are  stained  deeply  and  have  small  nuclei,  while  the  deeper  lying 
cells  are  larger,  more  lightly  stained,  and  have  larger  nuclei  and  more  distinct  cell 
boundaries.  The  mesoderm  consists  chiefly  of  somewhat  crowded  mesenchymal  cells, 
the  nuclei  of  which  are  smaller  than  the  entodermal  cells,  and  a  well-marked  layer  of 
mesothelium,  which  forms  the  external  covering  of  the  yolk-sac.  In  the  mesoderm 
appear  relatively  large  blood-vessels,  which  are  usually  found  filled  with  blood-cor- 
puscles. The  blood-vessels  have  distinct  endothelial  walls  and  lie  in  the  part  of 
the  mesoderm  toward  the.  mesothelium,  so  that  they  are  separated  somewhat  from 
the  entoderm  and  seem  often  to  lie  immediately  underneath  the  mesothelium.  They 
are  so  large  that  each  vessel  causes  a  protuberance  upon  the  yolk-sac. 


FIG.    260. — SECTION   OF  THE   YOLK-SAC 


CHAPTER  VIII. 

METHODS. 

Measuring  Length  of  Embryos. 

Owing  to  the  many  changes  during  development  in  the  curvature  of  the.  longi- 
tudinal axis  of  the  mammalian  embryo,  it  is  impracticable  to  measure  that  axis 
or  to  employ  any  one  system  of  measurements  to  obtain  comparable  results  for 
all  ages.  For  this  reason  the  best  practice  is  to  measure  in  all  cases  the  greatest 
length  of  the  embryo  in  its  natural  attitude  along  a  straight  line.  The  limbs  are 
not  to  be  included  in  such  measurements.  This  greatest  length  in  young  stages 
will  not  include  the  head  (compare,  for  example,  Fig.  95),  but  in  most  stages  the 
head  would  be  included.  Many  German  authors  employ  the  measurement  intro- 
duced by  His,  which  he  calls  the  Nackenldnge,  which  corresponds  to  the  distance 
in  a  straight  line  from  the  neck-bend  to  the  caudal  bend.  As  it  is  impossible  to 
measure  this  distance  in  later  stages,  it  seems  best  not  to  use  it  at  all.  The  length 
of  an  embryo,  as  given  by  German  authors,  is  often  indicated  by  the  abbreviation 
NL.,  and  is,  of  course,  often  different  from  the  measures  used  in  this  work. 

Dissection  of  Embryos. 

Vertebrate  embryos  may  be  dissected  without  serious  difficulty.  Specimens 
hardened  in  Zenker's  fluid  are  particularly  favorable  for  this  purpose.  Dissection 
should  be  more  extensively  practiced  than  is  at  present  usual  in  embryological  work, 
since  by  it  all  the  viscera,  the  central  nervous  system  and  even  the  main  nerve 
trunks  may  be  demonstrated  advantageously. 

By  the  following  simple  method  the  embryo  may  be  securely  attached  to 
the  bottom  of  the  dish  in  which  the  dissection  is  to  be  made,  best  in  80  per  cent 
alcohol.  Place  the  specimen  in  95  per  cent  alcohol  for  half  an  hour,  then  in  a 
mixture  of  equal  parts  of  alcohol  and  ether  for  fifteen  minutes.  Put  a  few  drops 
of  celloidin  dissolved  in  alcohol  and  ether  on  the  bottom  of  the  dish;  put  the 
specimen  in  place;  allow  the  celloidin  to  stand  for  two  or  three  minutes  until  a 
film  is  formed  and  then  cover  the  specimen  with  80  per  cent  alcohol,  which  will 
set  the  celloidin.  After  the  dissection  is  finished  the  specimen  may  be  freed  by 
dissolving  the  celloidin  with  a  mixture  of  alcohol  and  ether  in  equal  parts. 

Methods  of  Hardening  and  Preserving. 

The  three  most  generally  useful  methods  for  preserving  embryos  are  with  Zen- 
ker's or  Tellyesnicky's  fluids  and  10  per  cent  formalin.  Good  results  may  be  had 

377 


378  METHODS. 

with  the  other  reagents.  Specimens  preserved  with  Bouin's  fluid  have  the  advantage 
of  staining  sharply.  To  study  the  medullary  sheaths  of  nerve-fibers,  as  is  necessary 
to  follow  the  development  of  the  fiber  tracts  in  later  stages,  the  specimens  may 
be  preserved  in  Miiller's  fluid.  Flemming's,  Hermann's,  and  Bouin's  fluids  are  valua- 
ble, especially  for  cytological  study,  but  are  applicable  only  to  small  pieces. 

.1.  ZENKER'S  FLUID. 

Formula:      Corrosive  sublimate ...» 5  gm. 

Potassium  bichromate i  gm. 

Sodium  sulphate ' i  gm. 

Water 100  c.c. 

Add  5  c.c.  of  glacial  acetic  acid  to  the  fluid  immediately  before  using. 

The  fluid  does  not  have  great  penetrating  power,  but  may  be  used  for  embryos 
of  25  mm.  The  amount  of  fluid  should  be  from  ten  to  twenty  times  the  volume 
of  the  specimen,  and  better  results  are  obtained  if  the  fluid  is  changed  after  a  few 
hours.  Chicks  of  the  first  and  second  days  are  hardened  in  from  two  to  four  hours; 
embryos  of  from  6  to  8  mm.  in  from  eight  to  ten  hours;  embryos  of  12  mm.  in  twenty- 
four  hours;  larger  embryos  in  from  thirty  to  forty  hours.  After  the  proper  interval  in 
Zenker's  fluid  the  specimens  must 'be  removed  and  washed  in  running  water  for  from 
twelve  to  twenty-four  hours.  Transfer  to  50  per  cent  alcohol  for  from  one  to  three 
hours,  then  to  60  per  cent,  70  per  cent,  and  80  per  cent.  It  is  indispensable  to  remove 
now  the  excess  of  corrosive  sublimate  by  adding  sufficient  tincture  of  iodine  to 
give  the  alcohol  the  color  of  port  wine;  if  the  iodine  disappears,  it  must  be  renewed. 
After  from  one  to  three  days,  according  to  the  size  of  the  specimen,  transfer 
it  to  fresh  80  per  cent  alcohol,  which  must  be  changed  until  it  no  longer  extracts 
any  iodine  from  the  specimen. 

2.  TELLYESNICKY'S  FLUID. 

Formula:      Bichromate  of  potassium    3  gm. 

Water 100  c.c. 

Immediately  before  using  add  5  c.c.  glacial  acetic  acid. 

This  reagent  is  employed  in  the  same  manner  as  Zenker's  fluid,  except  that  the 
treatment  with  iodine  is  omitted. 

3.  FORMALIN. 

Formula:      Formalin    10  c.c. 

Water 7 90  c.c. 

Specimens  are  placed  in  the  fluid,  which  should  be  changed  in  a  few  hours.  On 
account  of  its  extreme  simplicity  this  method  is  used  especially  for  human  embryos. 
If  the  fluid  is  used  in  large  quantity  embryos  even  of  80  mm.  may  be  well  pre- 
served. They  may  be  kept  indefinitely  in  the  solution,  and  transferred  to  alcohol 
when  needed  for  sectioning.  The  method  has  the  disadvantage  that  the  transfer 
to  alcohol,  even  if  made  very  gradually,  always  causes  a  considerable  shrinkage. 


METHODS  OF  HARDENING  AND  PRESERVING.  379 

for  the  'prevention  of  which  no  satisfactory  method  has  been  devised.  Fortunately, 
the  shrinkage  usually  produces  no  distortion. 

4.  BOUIN'S  FLUID. 

Formula:      Picric  acid,  saturated  aqueous  solution 225  c.c. 

Formalin    ' 75  c.c. 

Glacial  acetic  acid ; , 15  c.c. 

Specimens  are  kept  in  the  fluid  from  two  to  seven  days,  not  longer,  according 
to  their  size;  transfer  to  30  per  cent  alcohol  for  one  hour,  to  50  per  cent  alcohol 
for  from  one  to  two  hours,  to  60  per  cent  alcohol  for  twelve  hours,  and  finally  to  70 
per  cent  alcohol,  which  must  be  changed  daily  until  it  no  longer  shows  even  a 
trace  of  yellow  discoloration  by  picric  acid. 

5.  MULLER'S  FLUID. 

Formula:      Bichromate  of  potassium 20  gm. 

Sulphate  of  sodium 10  gm. 

Water 1000  c.c. 

Miiller's  -fluid  is  a  valuable  reagent,  and  for  the  study  of  the  later  stages  of  the 
nervous  system  indispensable.  The  objections  to  its  use  are  that  it  requires  a 
long  time  to  act,  that  it  renders  the  specimens  brittle,  and  makes  them  somewhat 
difficult  to  stain.  It  must  be  used  in  large  quantities  and  be  frequently  changed, 
and  allowed  to  act  on  the  specimens  from  three  to  eight  weeks,  according  to  their 
size.  The  appearance  of  a  film  or  scum  indicates  that  the  fluid  needs  to  be  changed. 

6.  PARKER'S  FLUID. 

Formula:*    70  per  cent  alcohol .» 100  c.c. 

Formaldehyde i  c.c. 

Very  convenient  when  a  simple  and  expeditious  preservative  is  necessary.  The 
specimens  are  placed  in  the  fluid,  which  ought  to  be  renewed  in  a  few  hours. 
They  may  be  kept  permanently  in  the  fluid,  but  it  is  desirable,  before  using  them 
for  study,  to  remove  the  formaldehyde  by  treating  them  with  fresh  70  per  cent 
alcohol. 

7.  FLEMMING'S  FLUID. 

Formula:       i  per  cent  solution  of  chromic  acid .  .  .  .  • 50  c.c. 

2  per  cent  solution  of  osmic  acid j2  c.c. 

Glacial  acetic  acid 3  c.c. 

This  fluid  must  be  used  freshly  mixed,  and  must  not  be  kept  in  the  dark.  The 
specimens  must  be  of  small  size  and  as  fresh  as  possible.  The  amount  of  fluid 
used  should  be  not  less  than  15  times  the  volume  of  the  specimen.  Speci- 
mens are  kept  in  the  fluid  from  twenty-four  to  forty-eight  hours,  washed  in 
running  water  from  four  to  twenty-four  hours,  and  then  transferred  to  alcohols 
of  gradually  increasing  strength.  The  fluid  is  useful  chiefly  for  cytological  work. 

*  Differs  slightly  from  the  original  formula. 


380  METHODS. 

8.    HERMANN'S  FLUID. 

Formula:      i  per  cent  platinum  chloride  in  distilled  water 60  c.c. 

2  per  cent  osmic  acid  in  distilled  water 8  c.c. 

Glacial  acetic  acid 4  c.c. 

Used. in  the  same  manner  and  with  the  same  precautions  as  No.  7. 

Preservation  in  Alcohol. 

When  a  specimen  is  to  be  preserved  with  alcohol  alone,  it  should  be  put 
first  in  30  or  50  per  cent  alcohol  for  an  hour  or  more,  then  into  60  per 
cent  for  several  hours,  70  per  cent  for  from  twelve  to  twenty-four  hours,  and  finally 
into  80  per  cent,  in  which  it  should  be  kept  until  required  for  use.  If  the 
specimen  is  to  be  sectioned,  it  must  be  placed  in  95  per  cent  alcohol,  which  must 
be  renewed  at  least  once,  and  be  allowed  to  act  for  twenty-four  hours  or  more, 
unless  the  specimen  is  very  small,  when  a  somewhat  shorter  time  may  suffice. 

Directions  for  Imbedding  Specimens  to  be  Microtomed. 

A.  To  Imbed  in  Paraffin: 

1.  Stain  in  toto.      (See  page  382.) 

2.  Dehydrate  in  alcohol  from  three  to  twenty-four  hours. 

3.  Place    in    oil    of    cloves    and    turpentine    (equal    parts),    one    to    twenty-four 
hours. 

4.  Place  in  fresh  cloves  and  turpentine  for  one  to  twenty-four  hours. 

5.  Place  in  soft  paraffin  at   54°   C.   for  thirty  to  ninety  minutes. 

6.  Place  in  hard  paraffin  at  54°   C.   for  thirty  to  ninety  minutes. 

7.  Warm  a  glass    plate   to  about   70°   C.;  place  on  it  a  paper  tray  or  metal 
imbedding  frame;    fill   the  box  with   hard  paraffin'  at  54°  C.     Warm   a  spatula  and 
with  it  remove  the  specimen  to  the  tray  or  frame,  and  arrange  it  in  a  proper  posi- 
tion.     As  soon  as  the  paraffin   has  set,    chill  it  rapidly  with  cold  water,  otherwise 
the  paraffin  is  likely  to  crystallize  and  therefore  to  cut  badly. 

B.  To  Imbed  in  Celloidin: 

1.  Dehydrate    the    mass    thoroughly   in    95    per   cent   alcohol,    four   to    twenty- 
four  hours. 

2.  Place  mass  for  twenty-four  hours  in  alcohol  and  ether,  equal  parts. 

3.  Place  mass  in  thin  syrupy  solution  of  celloidin  in  equal  parts  of  ether  and 
alcohol   for  at  least  twenty-four  hours.      (If  the   specimen  contains   cavities   several 
days  are  necessary  to  allow  the  celloidin  to  penetrate  and  fill  them.) 

4.  Place  mass  in  thick  viscid  solution  of  celloidin  in  equal  parts  of  ether  and 
alcohol  for  twenty-four  hours. 

5.  Set  mass   on  block  of  vulcanite,   compressed   fiber,   or   maple-wood,   and   as 
soon  as  a  film  has  formed  over  the  surface  of  the  celloidin  (two  to  five  minutes)— 

6.  Immerse  in  80  per  cent  alcohol   for  twenty-four  hours. 

7.  Cut. 


METHODS  OF  STAINING.  381 

Method  of  Mounting  Paraffin  Sections. 

The  student  is  advised  to  use  for  serial  sections  slides  40  x  76  mm.  and 
from  1.75  to  2  mm.  thick.  The  thick  slides  are  much  better  than  the  thin 
ones  recommended  by  dealers.  The  cover-glasses  ought  to  be  0.17  to  0.18  mm. 
thick.  A  generally  convenient  size  is  35  x  50  mm. 

The  serial  order  of  the  sections  should  be  preserved  with  the  utmost  care, 
and  time  spent  in  arranging  the  sections  in  straight  rows  will  be  found  to  be  time 
saved. 

The  albumen-glycerin  method  of  fastening  the  sections  to  the  slide  will  be 
found  satisfactory. 

Formula:  Take  the  white  of  one  fresh  egg,  beat  slightly  until  equally  fluid,  filter  it  (it  will  take  about 
twenty-four  hours),  and  add  an  equal  amount  of  glycerin.  To  this  fluid  add  a  small  piece 
of  camphor. 

i.'   Clean  the  slide  thoroughly. 

2.  Put  on  a  small  drop  of  albumen  solution. 

3.  Spread  it  out  very  thin  with  the  finger. 

4.  Add  five   or  six  drops  of  distilled  water,  which  must  flow  evenly   over  the 
coating  of  albumen. 

5.  Place   the  sections  on  slides  in  regular  rows. 

6.  Warm    the    slides    gently   over   an    alcohol    flame,  to  allow  the    sections    to 
flatten.  The  paraffin  must  not  melt. 

7.  Drain  off    the  water- as  completely  as  possible,  and  arrange  the  sections  in 
straight  rows. 

8.  Place   the   slides   in  oven  for  twelve   to  twenty-four  hours   to  evaporate   the 
water  completely. 

9.  Dissolve  off  the  paraffin  in  turpentine    or    xylol. 

10.  Put  in  absolute  alcohol  for  three  to  five  minutes. 

11.  Clear  in  turpentine  or  chloroform. 

12.  Mount    in    damar   varnish.      (Canada    balsam    is    undesirable,    because    it 
becomes  much  discolored  with  age.) 

Methods  of  Staining. 

In  embryological  work  the  specimens  are  usually  stained  in  toto  before  im- 
bedding, either  with  alum  cochineal  or  with  borax  carmine,  the  former  being  the 
more  generally  useful  stain.  Staining  on  the  slide  is  also  much  used  either  to 
secure  a  counterstain  after  the  in  toto  coloration  or  to  secure  some  special  result. 
For  counterstains  eosin,  Lyons  blue,  and  orange  G  are  particularly  recommended. 
A  few  of  the  most  important  special  stains  are  given  below. 

i.  ALUM  COCHINEAL. 

Formula:      Powdered  cochineal 6  gm. 

Potassic  alum 6  gm. 

Water .80  c.c. 


382      .  METHODS. 

Boil  vigorously  for  twenty  minutes;  allow  the  fluid  to  settle;  decant  the  clear 
fluid;  add  more  water  and  boil  again;  filter  the  entire  solution  and  evaporate  the 
filtrate  to  80  c.c.;  add  a  small  piece  of  thymol  or  camphor  to  prevent  the  growth 
of  fungi.  Alum  cochineal  is,  on  the  whole,  the  best  reagent  for  in  toto  staining, 
as  it  will  penetrate  quite  large  objects  and  color  them  uniformly  throughout,  and 
gives  a  good  differentiation  of  the  tissues. 

For  in  toto  staining  place  the  specimen  in  water  until  it  sinks;  then  transfer 
it  to  the  cochineal  for  twenty-four  hours,  or  for  large  specimens  longer;  the  depth 
of  the  stain  will  depend  upon  the  strength  of  the'  solution;  transfer  to  clean  water 
for  fifteen  to  twenty  minutes  to  extract  the  alum,  which  otherwise  will  crystallize 
in  the  tissues  when  the  specimen  is  placed  in  alcohol;  the  object  must  not  be 
left  too  long  in  water,  because  it  extracts  the  color  also;  put  in  50  per  cent 
alcohol  for  one  hour,  then  successively  in  70  per  cent,  80  per  cent,  and  95  per 
cent,  when  the  specimen  will  be  ready  for  imbedding. 

2.  BORAX  CARMINE. 

Formula:      Best  carmine 3  gm. 

Borax 2  gm. 

Water 50  c.c. 

Boil  for  twenty  minutes;  allow  the  solution  to  cool;  'add  water  enough  to  restore 
that  lost  by  evaporation,  •  then  add  50  c.c.  of  70  per  cent  alcohol,  let  the  solution 
stand  twenty-four  hours;  filter.  Borax  carmine  gives  a  good  nuclear  stain  and 
may  be  advantageously  supplemented  by  counterstains. 

For  in  toto  staining  place  the  specimen  in  water  until  it  sinks;  transfer  to  the 
carmine  for  twenty-four  hours,  or  longer  for  large  specimens;  wash  in  water  for 
five  minutes;  then  place  it  in  70  per  cent  alcohol,  to  every  100  c.c.  of  which  2  c.c. 
of  hydrochloric  acid  have  been  added;  after  one  hour  transfer  to  fresh  70  per 
cent  alcohol,  which  must  be  renewed  in  an  hour  or  two,  and  finally  transfer  to 
80  per  cent  and  95  per  cent  alcohol,  and  the  specimen  will  be  ready  to  imbed. 

3.  ALUM  HEMATOXYLIN. 

Formula:      i.  Dissolve  10  grms.  hematoxylin  in  60  c.c.  absolute  alcohol. 

2.  Dissolve  15  grms.  ammonia  alum  in  100  c.c.  warm  water,  and  allow  the  solution  to  cool. 

3.  Mix  the  two  solutions  and  allow  the  stain  to  "ripen"  in  a  shallow  open  dish  exposed  to 

the  light  for  three  days. 

4.  Filter,  then  add  25  c.c.  pure  glycerin  and  25  c.c.  methyl  alcohol. 

5.  After  three  days  filter. 

The  solution  thus  prepared  will  keep  indefinitely.  It  is  one  of  the  best  nuclear 
stains  known.  Eosin  may  be  used  with  it  as  a  counterstain  giving  beautiful 
preparations. 

i.  Place  sections  from  water  into  a  filtered  diluted  solution  of  the  stain  for 
from  fifteen  minutes  to  twenty-four  hours,  according  to  the  strength  of  the  staining 
solution.  (Slow  staining  gives  the  best  results.) 


METHODS  OF  STAINING.  383 

2.  Wash   thoroughly  in   water  until   the   section   is   blue. 

3.  Dehydrate  and  mount. 

4.  COUNTERSTAINS    are    used    either    with    celloidin    sections    treated    singly,    or 
with  paraffin  sections  after  they  have  been  fastened  on  the  slide.      The  three  here 
recommended  are  alcoholic  solutions,  and  the  method  of  using  is  the  same  for  all. 
If  the  sections  are  dried  on  the  slide  in  an  oven  for  three  days,   they  will  adhere 
to    the    glass    much    more    securely    during    the    manipulations    of    counterstaining. 
Lyons  blue  is  less  permanent  than  eosin  and  orange   G. 

For  staining  paraffin  sections  on  the  slide  it  is  convenient  to  have  eight  jars 
or  dishes  large  enough  to  hold  a  slide.  The  slide  is  transferred  from  jar  to  jar 
in  the  order  below,  being  allowed  to  remain  in  each  jar  a  few  minutes.  The 
very  most  scrupulous  care  is  necessary  to  keep  all  the  fluids  clean,  and  it  is  indis- 
pensable to  filter  them  frequently;  the  sections  on  the  slide  catch  and  hold  the 
particles  floating  in  the  reagents  when  they  are  not  clean. 

Order  of  jars:     i.    Xylol. 

2.  Xylol. 

3.  Xylol   and   absolute  alcohol,   equal   parts. 

4.  Absolute  alcohol. 

5.  Counterstain. 

6.  Alcohol  of  95   per  cent. 

7.  Absolute  alcohol. 

8.  Xylol. 


Eosin  Formula:  2  per  cent  in  95  per  cent  alcohol. 

Orange  G.  Formula:       i  per  cent  in  95  per  cent  alcohol. 
Lyons  blue  Formula:        i  per  cent  in  95  per  cent  alcohol. 


5.    HEIDENHAIN'S   IRON  HEMATOXYLIN. 

Formula  I:  Iron  alum    2  gm. 

Distilled  water 100  c.c. 

II:  Hematoxylin  crystals    i  gm. 

95  per  cent  alcohol 10  c.c. 

Distilled  water,  to  be  added  after  the  hematoxylin  is    dissolved  in  the 

alcohol , 90  c.c. 

(If  the  stain  does  not  work,  add  0.5  grm.  lithium  bicarbonate.) 

1.  Place  sections   in    the  iron   solution   for   from    six   to  ten  hours.     (Specimens 
hardened  with  Flemming's  or  Hermann's  fluid  require  longer  than   specimens  from 
Zenker's    or  Tellyesnicky's  fluid.) 

2.  Wash   quickly  in  water. 

3.  Transfer  to  the  hematoxylin  solution   for  from  twtlve    to    eighteen    hours. 

4.  Wash  in  tap-water. 

5.  Decolorize  in   the  iron   solution. 

6.  Wash  thoroughly  in  tap-water. 

7.  Dehydrate  and  mount. 


384  METHODS. 

This  stain  is  useful  for  cytological  work,  the  study  of  cell  division,  etc.  The 
preparations  are  often  improved  by  counterstaining  with  orange  G. 

6.  BEALE'S  CARMINE. 

Formula:      Best  carmine i  gm. 

Ammonia v 3  c.c. 

Pure  glycerin 96  c.c. 

Distilled  water    96  c.c. 

Alcohol,  95  per  cent 24  c.c. 

Dissolve  in  ammonia  plus  part  of  the  water,  add  the  rest  of  the  water,  and 
allow  the  solution  to  stand  in  an  open  dish  until  the  ammonia  is  nearly  all  driven 
off.  Then  add  the  alcohol  and  glycerin.  For  use  dilute  with  an  equal  part  of 
glycerin.  Stain  for  twenty-four  hours  in  an  open  dish,  which,  together  with  a 
second  open  dish  containing  acetic  acid,  is  placed  under  a  bell- jar;  wash  the 
sections  thoroughly  in  water  and  then  in  very  weak  hydrochloric  acid  (i  c.c.  to 
500  c.c.  water),  and  again  in  water. 

Beale's  carmine  is  especially  valuable  for  the  study  of  the  central  nervous 
system  and  of  the  placenta. 

7.  WEIGERT'S  COPPER  HEMATOXYLIN. 

Formula  I:  Copper  solution: 

Acetate  of  copper  in  saturated  aqueous  solution. 
II:  Hematoxylin  solution: 

Hematoxylin  crystals 2  gm. 

95  per  cent  alcohol 20  c.c. 

Distilled  water    80  c.c. 

(If  the  stain  does  not  work,  add  0.5  grm.  of  lithium  bicarbonate.) 
Ill:  Iron  solution: 

Ferricyanide  of  potassium 25  gm. 

Borax 20  gm. 

Water 2000  c.c. 

This  stain  is  indispensable  for  the  study  of  the  nervous  system  after  the 
medullary  sheaths  have  begun  to  develop;  the  specimens  must  be  preserved  in 
Miiller's  fluid.  The  method  is  also  valuable  for  the  study  of  the  placenta  and 
uterus. 

1.  Place  the  sections  in  water. 

2.  Place  the  sections  in  the  copper  solution  for  twenty-four  hours. 

3.  Wash  quickly  in  water. 

4.  Put  them   in   the   hematoxylin   solution   for  five   to   ten   minutes.      The   sec- 
tions should  turn  a  deep  blue-black. 

5.  Wash   thoroughly   in  water. 

6.  Decolorize   in    the   iron   solution;    the    section   must   be    gently   moved    about 
to  secure  an  even  decolorization.     When  part  of  the  section  shows  a  brown  color, 
it  should  be  removed  and  examined. 


METHODS  OF  RECONSTRUCTION.  385 

7:  Wash  thoroughly  in  water  to  remove  the  iron  solution,  no  trace  of  which 
can  be  left  without  ruining  the  specimen.  (Unless  the  washing  is  very  thorough, 
the  sections  will  gradually  fade  out  after  the  final  mounting.) 

8.  Dehydrate  with  alcohol  and  mount  at  once  in  damar. 

8.  MALLORY'S  TRIPLE  CONNECTIVE-TISSUE  STAIN. 

Formula  I:  Acid  fuchsine    i .  o  gin. 

Distilled  water    1000 .  o  c.c. 

II:   Distilled  water    100.0  c.c. 

Phosphomolybdic  acid,  to  be  added  before  other  ingredients    i .  o  gm. 

Aniline  blue,  soluble  in  water 0.5  gm. 

Orange  G 2.0  gm. 

1.  Preserve  in  corrosive  sublimate  or  Zenker's  fluid. 

2.  Stain  the  sections  in  the  fuchsine  solution  one  to  three  minutes. 

3.  Wash  in  water  very  quickly. 

4.  Place  in  the  phosphomolybdic  solution  two  to  twenty  minutes. 

5.  Wash  off  excess  stain  in  water. 

6.  Dehydrate  in  95  per  cent  alcohol  and  mount  in  damar. 

This  method  gives  a  perfect  differential  stain  of  connective-tissue  fibrils,  and  it 
is  to  be  used  whenever  the  fibrils  are  to  be  especially  studied. 

Methods  of  Reconstruction. 

It  is  often  important  to  obtain  definite  plastic  conceptions  of  the  anatomy 
of  embryos  or  parts  of  embryos  too  small  for  dissection.  To  secure  these  in  the 
best  form,  it  is  necessary  to  reconstruct  either  drawings  or  models  from  sections. 
The  methods  employed  for  these  two  forms  of  reconstruction,  being  different,  must 
be  described  separately. 

Reconstruction  of  Drawings  from  Sections. — To  make  these  reconstructions 
satisfactory,  it  is  indispensable  to  have  an  accurate  outline  of  the  embryo  repre- 
senting it  in  the  point  of  view  to  be  used  for  the  reconstruction  and  enlarged  to 
the  precise  scale  upon  which  the  reconstruction  is  to  be  made.  This  drawing 
must,  of  course,  be  made  before  the  embryo  is  imbedded  and  sectioned.  It  is 
further  necessary  to  know  accurately  the  plane  of  the  sections  and  their  thickness, 
and,  finally,  the  total  number  of  sections  in  the  series  must  be  counted.  A  con- 
venient scale  for  the  reconstruction  of  the  anatomy  of  mammalian  embryos  is  a 
magnification  of  from  15  to  30  diameters. 

Let  us  suppose  that  a  pig  of  12  mm.  has  been  drawn  in  a  side  view  magnified 
20  diameters;  that  the  embryo  has  been  cut  into  900  transverse  sections  and  the 
approximate  plane  of  the  sections  is  known.  It  may  be  more  exactly  determined 
by  the  study  of  the  sections  themselves;  for  instance,  it  may  be  determined 
what  section  is  the  last  to  pass  through  the  surface  of  the  head  in  the  region  of  the 
fore-brain  and  the  last  to  pass  through  the  border  of  the  anterior  limb.  Then  it 


386  METHODS. 

can  be  further  ascertained  through  which  dorsal  segments  these  two  sections  pass. 
By  these  data  the  plane  of  the  two  sections  can  be  accurately  fixed.  Over  the 
outline  of  the  embryo  is  now  drawn  a  series  of  lines  which  represent  the  position 
of  the  sections.  It  is  generally  sufficient  to  put  in  lines  which  represent  only  every 
second,  third,  or  even  fourth  section.  If  at  any  point  where  the  structure  is  com- 
plicated more  details  are  needed,  lines  for  the  additional  sections  can  be  interpolated. 
In  our  supposed  case,  our  lines  representing  every  fourth  section,  there  would  be 
225  parallel  lines,  and  these  should  be  numbered  to  correspond  to  the  sections 
which  they  represent. 

The  outlines  of  the  actual  sections  corresponding  to  the  numbered  lines  in 
the  diagram  must  now  be  made  with  the  camera  lucida.  In  regard  to  these  great 
care  is  necessary,  especially  if,  as  is  likely  to  be  the  case,  the  sections  are 
from  embryos  imbedded  in  paraffin,  because  when  an  embryo  is  so  imbedded  it 
always  shrinks,  and  after  imbedding  is  smaller  than  before.  The  shrinkage  seems 
to  be  uniform  throughout  and  not  to  disturb  the  topographical  relations  even  of 
the  finest  structures.  Unfortunately  the  shrinkage  is  not  constant,  but  varies  from 
specimen  to  specimen,  hence  a  camera  drawing  made  from  the  sections  and  magni- 
fied 20  diameters  will  not  be  of  the  right  size  to  fit  in  the  diagram,  and  these 
drawings,  must,  therefore,  be  corrected.  This  may  be  done  either,  as  is  best,  by 
making  the  original  camera  lucida  drawings  of  the  right  magnification  for  direct 
use  in  reconstruction,  or  they  may  be  made  nearly  the  right  magnification  and 
when  they  are  measured  off  the  necessary  correction  may  be  introduced  by  measur- 
ing them  with  proportional  dividers. 

From  the  camera  lucida  drawings  of  the  single  sections  the  measurements  are 
taken  to  fix  the  position  of  the  parts  in  the  reconstruction. 

For  a  given  section  the  exact  position  in  the  reconstruction  is  fixed  by  the  line 
on  the  outline  drawing  of  the  embryo  corresponding  to  the  number  of  the  section. 
On  the  drawing  of  the  section  the  distance  of  the  organ  to  be  reconstructed  from 
the  point  in  the  section  corresponding  to  the  outline  of  the  embryo  is  measured  off, 
and  then  marked  upon  the  proper  line  of  the  reconstruction  diagram.  A  similar 
measurement  is  then  taken  from  the  next  section  and  transferred  to  the  diagram  in 
the  same  manner,  and  so  on .  with  successive  sections  until  a  series  of  dots  is 
obtained  which  mark  the. outline  of  the  organ.  These  dots  are  then  connected  by 
a  continuous  line,  .which  will  indicate  the  form  and  correct  position  of  the  organ. 
Simple  reconstructions  may  be  easily  made  by  these  means.  When,  however,  more 
complicated  reconstructions  are  attempted,  much  judgment  and  skill  are  necessary 
in  the  selecting  of  parts  which  may  be  successfully  represented  in  a  single  drawing, 
bearing  in  mind  always  the  point  of  view  which  is  assumed  for  the  reconstruction, 
so  that  organs  may  be  correctly  represented  in  their  relative  positions,  nearer  or 
farther  from  the  observer  as  he  looks  at  the  drawing  .  After  the  outlines  are 
completed  the  shading  of  the  parts  must  be  added,  and  this  often  requires  a  special 
degree  of  skill  and  a  considerable  faculty  of  plastic  imagination.  As  examples  of 


METHODS  OF  RECONSTRUCTION.  387 

complicated   reconstructions,  the   student   is  referred   to   figures    169   and    172,   pages 
229  and  235. 

Oftentimes  simpler  reconstructions  are  very  helpful  in  which  only  a  few  sections 
are  combined,  as,  for  example,  to  show  the  course  and  branches  of  the  spinal 
nerves  in  young  embryos.  In  such  a  case  the  outline  of  the  middle  section  of 
the  series  proposed  to  be  combined  may  be  selected  to  give  the  outline  of  the 
reconstructed  drawing.  Camera  lucida  drawings  of  this  and  the  neighboring  sec- 
tions to  be  included  should  be  made  of  the  desired  magnification.  The  reconstruc- 
tion itself  may  be  made  upon  tracing  paper,  which  is  laid  successively  over  the 
drawings  of  the  sections  and  the  parts  required  from  each  can  be  added  upon  the 
tracing  paper,  which  will  thus  combine  in  a  single  drawing  the  parts  intended  to 
be  represented.  Reconstructions  of  this  kind  are  easily  made  by  students  and  are 
often  very  instructive. 

Reconstruction  with  Wax  Plates  by  Barn's  Method. — The  basis  of  this  method 
is  to  make  in  wax  a  magnified  reproduction  of  the  single  sections,  representing  in 
the  wax  such  portions  of  the  section  as  it  is  desired  to  reproduce  in  plastic  recon- 
struction. To  this  end  wax  plates  must  be  made  which  represent  a  definite  magni- 
fication of  the  thickness  of  a  section.  For  working  by  this  method  it  is  usually 
advantageous  to  employ  rather  thick  sections,  say,  of  20^.  If  the  magnification 
chosen  is  fifty  times,  which  is  practically  often  convenient,  then  the  wax  plates 
should  be  made  fifty  times  2o/i  in  thickness,  or  i  mm.  The  most  convenient 
plates  to  work  with  are  those  from  i  to  2  mm.  thick.  Upon  a  wax  plate  of  the 
requisite  thickness  a  camera  lucida  drawing  is  made.  This  may  be  done  with  a 
lithographic  crayon  or  with  a  fine  steel  point.  The  drawings  must  be  of  exactly 
the  right  magnification;  in  the  illustration  chosen,  50  diameters.  Next,  the  wax  plate 
is  put  upon  a  glass  or  a  metal  surface  where  it  lies  perfectly  flat,  and  with  a 
sharp  thin-bladed  knife  or  scalpel  the  outlines  of  the  organs  which  it  is  intended 
to  reconstruct  are  cut  out  as  may  be  desired.  Our  bit  of  wax  then  represents  a 
model  of  the  parts  selected  from  the  section,  and  equally  magnified  in  the  three 
dimensions  of  space.  Wax  plates  made  from  successive  sections  are  then  piled 
up,  one  on  top  of  the  other,  in  the  proper  order.  If  they  are  rightly  superimposed, 
an  operation  which  often  requires  skill  and  judgment,  and  always  requires  the 
utmost  care,  then  the  pile  of  plates  will  correctly  represent  the  form  of  the  parts 
included  in  the  reconstruction.  To  fasten  the  plates  together  it  is  only  necessary 
to  pass  a  warm  metal  instrument  over  the  edges  of  the  plates,  enough  to  melt  the 
wax  a  little.  With  proper  care  this  may  readily  be  accomplished  without  destroy- 
ing the  surface  modeling  of  the  reconstruction. 

The  simplest  method  of  making  wax  plates  is  to  have  a  large  tin  pan  with 
vertical  sides.  This  is  filled  with  very  hot  water,  and  melted  beeswax  is  poured  on 
the  surface  of  the  water  and  allowed  to  cool.  Plates  of  sufficiently  exact  and  even 
thickness  may  be  cast  in  this  way,  provided  the  operation  is  carried  out  in  a  quiet 
place  so  that  the  surface  of  the  water  is  not  disturbed  while  the  wax  is  hardening. 


388  METHODS. 

It  will  be  found  convenient  to  have  a  large  plate  of  iron,  not  less  than  one  eighth  of 
an  inch  in  thickness,  which  may  be  placed  upon  supporters.  The  tin  pan  should  be 
set  upon  this  plate  and  the  plate  heated  by  lamps  below  in  order  to  keep  the  water 
hot  enough  to  allow  the  wax  to  spread  evenly  over  the  surface  of  the  water.  The 
water  must  be  freed  from  air  before  the  wax  is  poured  in,  but  must  not  be  allowed  to 
boil  after  the  wax  has  been  added.  If  bubbles  appear  in  the  wax  plate,  they  may  be 
removed  while  the  wax  is  still  hot  by  directing  the  blue  flame  from  a  Bunsen  burner 
down  upon  them.  If  the  pan  is  heated  directly  without  the  iron  plate,  it  is  sure 
to  warp  and  become  unfit  for  use.  Thin  iron  plates  are  also  liable  to  be  warped. 
To  determine  the  thickness  of  the  plates  cast  as  described  we  proceed  em- 
pirically. A  weighed  quantity  of  wax  is  melted  and  poured  into  the  pan.  After 
the  plate  has  solidified  it  is  removed  by  cutting  it  free  from  the  edges  of  the  pan, 
and  the  thickness  of  the  plate  is  then  measured  at  various  points  by  micrometer 
callipers.  From  these  data  it  is  easy  to  calculate  exactly  what  thickness  of  plate 
one  gram  of  beeswax  represents.  To  get  accurate  results  it  is  advisable  to  cast 
several  plates  of  varying  thickness  and  determine  the  average  for  one  gram  in 
that  way.  Having  determined  what  one  gram  represents  in  thickness,  it  becomes 
thereafter  only  necessary  to  weigh  out  the  proper  number  of  grams  in  order  to 
obtain  any  desired  thickness  of  wax  plate.  It  will  be  found  advantageous  to  filter 
the  wax  before  using  it.  This  may  easily  be  done  by  a  double  hot-water  filter. 
Such  a  filter  may  be  made  of  copper.  It  is  desirable  to  connect  it  with  a  Mariotti's 
flask  to  maintain  a  constant  water  level. 

Directions  for  Orienting  Serial  Sections  of  Embryos.  (NOTE:  The  lower 
edge  of  the  ribbon  is  the  one  to  the  left,  when  the  observer  has  the  object  between 
himself  and  the  knife.) 

1.  Transverse  Series. 

Normal   thickness:  io//. 

Dorsal  surface  to  be  toward  the  lower  edge  of  the  ribbon. 
Series   to   begin   with   the   head. 
In  cutting,  the  left  side  of  the  embryo  must  strike  the  knife  first. 

2.  Sagittal  Series. 

Normal  thickness:  Small  embryos,  io/*. 
Medium  "  i5//. 
Large  20^. 

The  head  of  the  embryo  to  be  toward  the  lower  edge  of  the  ribbon. 
Series  to  begin  with  the  right  side. 

In  cutting,  the  ventral  side  of  the  embryo  must  strike  the  knife  first. 
^3.  Frontal  Sections. 

Normal  thickness:  Small  embryos,  iofi. 
Medium  "  15;*. 
Large  20;*. 


MICROTOMES. 


389 


the 


The  head  of  the  embryo  is  to  be  toward  the  lower  edge  of  the  ribbon. 

The  series  is  to  begin  with  the  ventral  side. 

In  cutting,  the  left  side  of  the  embryo  must  strike  the  knife  first. 

In  mounting  leave  space  for  the  label  at  the  left-hand  end  of  the  slide.     Keep 

sections    in    the    order   cut.     Arrange    them    on    the    slides    in    the    sequence    of 


ordinary  written  lines. 

Microtomes. 

There  are  many  forms  of  microtome  which  may  be  used  with  good  results 
and  which  will  work  very  satisfactorily  for  making  sections  of  small  objects.  The 
cutting  of  larger  objects,  such  as  pig  embryos  of  from  15  to  20  mm.,  and  of  pieces 


FIG.  261. — THE  PRECISION  MICROTOME. 

of  the  uterus  or  other  organs,  is  more  difficult,  and  microtomes  which  work  satis- 
factorily with  small  objects  often  fail  to  give  good  even  sections  of  more  difficult 
objects.  For  embryological  work  a  microtome  ought,  therefore,  to  be  selected 
which  will  give  perfectly  regular  sections  in  long  series  of  any  desired  thickness 
from  i/*  up  to  25/4.  It  is  also  desirable  for  economy  of  time  to  have  a  micro- 
tome which  works  automatically.  These  considerations  lead  the  author  to  recom- 


390 


METHODS. 


mend  for  embryological  use  especially  two  forms  of  microtome  made  by  Messrs. 
Bausch  &  Lomb,  of  Rochester,  N.  Y.,  and  designated  by  them  as  the  "precision" 
and  " rotary"  microtomes. 

The  precision  microtome  (Fig.  261)  consists,  first,  of  an  upper  square  form 
upon  which  the  knife  may  be  clamped  in  any  desired  position;  second,  of  two 
horizontal  ways  upon  which  moves  the  carriage  which  bears  the  object-holder; 
and,  third,  of  a  micrometer  screw  with  an  automatic  feeding  contrivance  on  the 
under  side  of  the  movable  carriage.  The  construction  is  very  solid  and  great 
rigidity  of  the  parts  is  secured.  The  microtome  may  be  used  for  either  paraffin 
or  celloidin  cutting.  According  to  the  author's  experience,  this  microtome  con- 
siderably surpasses  all  other  types  in  the  accuracy  of  the  work  which  may  be  done 


FIG.  262. — THE  AUTOMATIC  ROTARY  MICROTOME. 


with  it.  The  rotary  microtome  was  originally  made  in  Germany,  and  various 
patterns  have  been  put  upon  the  market  by  German,  French,  English,  and  Amer- 
ican manufacturers.  The  new  pattern  recently  introduced  by  Messrs.  Bausch  & 
Lomb  embodies  a  considerable  number  of  improvements,  which  render  the  instru- 
ment (Fig.  262)  very  desirable  for  general  laboratory  use.  It  works  with  accuracy, 
is  very  easy  to  manipulate,  and  cuts  sections  with  extreme  rapidity.  It  is  adapted 
only  for  paraffin  work.  For  the  general  use  of  students,  in  elementary  courses 
especially,  this  microtome  is  to  be  preferred  to  the  "  precision,"  as  it  requires 
less  care  and  works  more  rapidly.  A  single  rotary  microtome  will  be  found 
sufficient  for  a  class  of  from  twenty  to  thirty  students  in  embryology. 


MICROTOMES.  391 

The  microtome  is  an  instrument  of  precision,  which  implies  that  it  must  be 
treated  with  extreme  delicacy  and  kept  most  scrupulously  clean.  It  will  be 
found  usually,  when  complaint  is  made  against  the  microtome,  that  the  complaint 
is  misdirected,  and  ought  to  be  not  against  the  machine,  but  against  the  owner. 
The  modern  microtome  necessarily  has  several  adjustments,  every  one  of  which 
must  be  exact  and  secure.  If  any  one  of  them  is  imperfect  or  insecure,  if  any 
of  the  movable  parts  is  allowed  to  become  corroded,  or  gummed  up  with  oil,  or 
loose,  or  clogged  with  dust  or  dirt  of  any  kind,  the  microtome  will  not  and  cannot 
work  as  an  instrument  of  precision. 

The  knife  used  for  cutting  ought  to  be  regarded  as  an  integral  part  of  the 
microtome  and  as  its  most  delicate  and  easily  injured  part.  A  perfect  knife-edge 
is  the  greatest  treasure  of  the  microtomist.  To  sharpen  the  knife  satisfactorily 
for  fine  section  cutting  is  a  really  serious  difficulty.  A  skillful  person,  however, 
may  get  a  good  edge  by  using  the  very  finest  grade  of  oil-stone.  No  oil  should 
be  used,  but  instead  a  mixture  of  equal  parts  of  glycerin  and  water.  Before  the 
knife  is  honed  it  must  be  made  as  clean  as  possible.  The  oil-stone  itself  also 
must  be  cleaned  with  equal  care,  and  the  mixture  of  glycerin  and  water  should, 
if  necessary,  be  filtered  before  using  to  keep  it  free  from  dirt.  A  single  particle 
of  dirt  may  be  the  cause  of  making  many  microscopic  notches  in  the  edge  of 
a  knife.  A  knife  is  well  sharpened  when  its  edge  appears  smooth  and  straight 
under  a  magnifying  power  of  twenty-five  diameters.  The  microtome  knife  should 
be  as  unlike  a  razor  as  possible.  It  must  have  a  very  thick  back  and  be  as 
heavy  and  rigid  as  practicable,  so  that  the  actual  cutting-edge  may  be  as  steady 
and  inflexible  as  it  can  be  made.  Knives  of  suitably  heavy  construction  are  now 
furnished  with  all  the  best  microtomes. 


INDEX. 


Abdominal  cavity,  origin,  87 

Abozzo,  9 

After-birth,  denned,  360 

Alcohol,  380 

Allantois,  chick,  214 

general  account,  no 

human,  136,  142,  374 

in  umbilical  cord,  374 

origin,  83 

in  primates,  123 

pig,  9.0  mm.,  253 

17.0  mm.,  310,  311 

and  chorion  in  ungulates,   113 
Alum  cochineal,  381  . 

hematoxylin,  382 
Amiurus,  51 

Amnio-cardiac  vesicles,  chick,  180,  186 
Amnion,  general  account,  117 

human,  at  two  months,  370 
at  three  months,  344 
after  fifth  month,  371 
at  seven  months,  345 
structure,  370 

origin,  83 

in  primates,  122 

raphe,  chick,  203,  210 
Amniota,  7 
Anal  plate,  58,  59 

origin  of,  54 
Anamniota,  7 
Angioblast,  chick,  187 

development,  91 

early  history,  80 

first  appearance,  66 
Anlage,  defined,-  9 
Annelids,  9 
Anterior  cavity,  88 
Anura,  8 

Anus,  origin  of,  54 
Aorta,  chick,  191,  192,  200,  202,  205 


Aorta,  descending,  pig,  6.0  mm.,  247 

12  .  o  mm.,  274,  281 
dorsal,  pig,  9.0  mm.,  255 

12. o  mm.,  282,  283,  285 
17.0  mm.,  305 
first  appearance,  93 
Aortic  arches,  chick,  184,  202 
general  account,  100 
human,  142,  144,  145,  146 
pig,  6.  o  mm.,  247 

12. o  mm.,  278,  293 
Zimmermann's,  100 
Aortic  system,  general  account,  99 
Appendages,  embryonic,  absence  in  lower  verte- 
brates, 82 

Arachnoid,  pig,  9.0  mm.,  254 
12  .o  mm.,  266 
17.0  mm.,  305 
20. o  mm.,  325 
Archenteron,  general  account,  57 

origin  of,  54 
Area,  germinal,  96 

opaca,  defined,  64 

first  appearance,  97 
pellucida,  defined,  64 

first  appearance,  97 
vasculosa,  50,  66,  91 

first  appearance,  97 
of  rabbit,  98 
vessels  of,  97 
vitellina,  65 
Arteries,  basilaris,  pig,  9.0  mm.,  258 

12  .o  mm.,  264,  293 
carotid,  pig,  12  mm.,  235,  271,  274 

loop,  pig,  264 
central  of  retina,  333 
intersegmental,  pig,  6.0  mm.,  249 
9.0  mm.,  250 
12  .  o  mm.,  301 
lingual,  329 


393 


INDEX. 


Arteries,  posterior  communicating  branch,  265 
pulmonary,  pig,  12.0  mm.,  281 

20.  o  mm.,  317 
sulci,  312,  320 

umbilical,  pig,  17.0  mm.,  310,  311 
vertebral,  pig,  12.0  mm.,  273 

24 .  o  mm.,  336 
vitelline,  311 

chick,  211 

Ascending  trigeminal  tract,  263 
Astral  figures  in  ovum,  40 
Atriozoa,  9 
Auricle,  pig,  12.0  mm.,  282 


Bacilliform  bodies,  168 
Beale's  carmine,  384 
Biogenetic  law,  3 1 
Bladder,  origin,  114 
Blastodermic  vesicle,  45 

formation  of,  44 

in  primatesl  122 

in  monkey,  second  stage,  128 

in  rabbit,  166 

study  of,  167-173 
Blastopore,  53 

division  of,  54 
Blood,  degeneration  in  chorion,  357 

origin,  90 

pig,  12  .  o  mm.,  266 
20  .  o  mm.,  324 
Blood-corpuscles,  93 

red,  general  account,  93 
ichthyoid,  94 
sauroid,  94 
mammalian,  94 
Blood-islands,  91 

chick,  179 
Blood-plates,  94 
Blood-vessels,  definition,  90 

development  in  chick,  90 
in  mammals,  92 

first  appearance,  66 

growth  into  embryo,  93 

origin,  90 

primitive  course,  man,  142 
Body-cavity  of  vertebrates,  5 
Body-stalk,  defined,  1 1 1 

origin  in  primates,  123 

vessels  of,  112 

and  umbilical  cord,  1 1 5 
Borax  carmine,  382 
Bern's  method,  387 
Brain,  73 


Branchial  arches,  defined,  62 
pig,  6.0  mm.,  246,  249 
7  .  8  mm.,  222 
9  .  o  mm.,  258 
10.  o  mm.,  224 
12  .  o  mm.,  274 


Canal,  auricular,  283 

digestive,  59 

hyaloid,  333 

neurenteric,  origin  of,  54,  68 

notochordal,  53 

of  Schlemm,  331 
Capsule,  periotic,  78 
Carnivora,  8 
Carotids,  origin,  100 
Cartilage,  first  appearance,  304. 

Meckel's,  329 
Cavity,  abdominal,  origin,  87 

anterior,  88 

head,  87 

hyoid,  87 

mandibular,  87 

pericardial,  chick,  181,  209 
origin,  87 

pleural,  87 

premandibular,  87 
Cells,  death  of,  15' 

germ,  25 

lutein,  35 

removal  of,  16 

somatic,  28 
Cephalochorda,  9 
Cerebellum,  pig,  12.0  mm.,  292 
Cervical  sinus,  human,  147—149 
Cheiroptera,  8 
Chick  embryo,  method  of  obtaining,  i  74 

preservation,  176 

study  of,  174-218 

with  eight  segments,  176 

comparison  with  rabbit,  179 
longitudinal  section,  180 
transverse  sections,  181 

with  twenty-four  segments,  197 

with  twenty-eight  segments,  differentiation 

in,  216 

studied  in  sections,  199-216 
Chondrostyle,  56 

pig,  24.0  mm.,  336 
Chorda  dorsalis.     See  Notochord. 
Chorion,  chick,  203,  211 

defined,  64 

frondosum,  defined,  127 


INDEX. 


395 


Chorion,  general  account,  1 1  7 

human,  at  three  months,  344 

degeneration  of  blood  in,  3^7 
ectoderm  of,  364 
histology  of,  363 
laeve,  second  stage,  350 
mesoderm  of,  356,  364 
placental,  354 
trophoderm  of,  365 
vijli,  367 

laeve,  denned,  127 

origin,  83 

relation  to  uterus  in  ungulates,  113 
Chromosome,  accessory,  28 

number  in  segmentation  nucleus,  41 
Cinerea,  263,  270 

defined,  74 
Cisterna  chyli,  105 
Cochlea,  origin,  78 
Coelom,  defined,  18 

double,  8 1 

embryonic,  84 

of  the  head,  87 

origin,  81 

pig,  9.0  mm.,  255 
12  .o  mm.,  301 

umbilical,  311 

ventral,  87 

vertebrates,  5 
Commissure,  ganglionic,  262 

posterior,  335 

superior,  335 
Copper  hematoxylin,  384 
Cord,  umbilical,  pig,  17.0  mm.,  310 

sexual,  322 
Corona  radiata,  34 

Corpora  quadrigemina,  pig,  12.0  mm.,  292, 
Corpus  luteum,  35 

striatum,'  329 
Costal  processes,  305,  321 
Counterstains,  381 
Cutis,  anlage,  266 

pig,  20.  o'  mm.,  321 
Cutis-plate,  86 
Cytomorphosis,  1 1 


Decidua  caduca,  defined,  124 
cavernous  layer  of,  351 
compact  layer  of,  351 
defined,  124 

reflexa,  at  three  months,  343 
at  four  months,  344 
•atrophy,  127 


Decidua  reflexa,  defined,  124 
disappearance,  343 
first  stage,  349 
serotina,  at  seven  months,  345,  357 

defined,  124 
subchorialis,  359 
vera,  at  three  months,  344 
defined,  124 
first  stage,  346 
second  stage,  350 
Decidual  cells,  development,  348 

mature,  359 
Deck-plate,  72 

pig,  9.0  mm.,  257 
Degeneration,  hypertrophic",  15 
Dermatome,  86 
Development,  arrest  of,  91 
embryonic,  16 
larval,  16 
summary  of,  10 

Diaphragm,  pig,  24.0  mm.,  337 
Diencephalon,  origin,  74 
Differentiation,  13 

two  types  of,  14 
Digestive  canal,  general  account,  59 

of  vertebrates,  5 
Dipnoi,  8 

Discus  proligerus,  34 
Diverticulum,  Meckel's,  57 
Dorsal  furrow,  68 

roots,  270,  283 
Ducts,  Miillerian,  no,  317 
urogenital,  no 
Wolffian,  no 
chick,  212 
pig,  12  .  o  mm.,  287 
17.0  mm.,  309 
20.  o  mm.,  318 
Ductus  arteriosus,  91 
defined,  101 
Cuvieri,  defined,  103 
endolymphaticus,  299 

origin,  78 
thoracicus,  105 

venosus  Arantii,  pig,  9.0  mm.,  252 
Dyads,  37 


Ebauche,  9 
Ectoderm,  chick,  217 

defined,  18 
Ectoglia,  263,  314 

defined,  74 
Elasmobranchs,  8 


INDEX. 


Embryo,  Amiurus,  51 

appendages  of,  49 

arising  from  embryonic  shield,  50 

dissection,  377 

growth  of,  49 

human,  118 

imbedding,  380 

measuring,  377 

preservation,  377 

separation  from  yolk,  49 
Embryonic  shield,  44,  47 

rabbit,  170—173 
Entoderm,  chick,  217 

denned,  18 

earliest  growth,  mammals,  46 

origin  of  permanent,  54 
Eosin,  383 

Ependyma,  defined,  74 
Ependymal  layer,  primitive,  263 
Epidermis,  pig,    12.0  mm.,  266 

17.0  mm.,  303 
Epiglottis,  337 

pig,  12.0  mm.,  293 
Epiphysis,  335 
Epithelial  bodies,  63 
Epitrichium,  303,  320 
Eustachian  tube,  origin,  63 
Excretory  organs,  general  account,  108 
Eye,  general  account,  76 

pig,  12  .o  mm.,  271 
29. o  mm.,  329 
24.0  mm.,  331 
Eyelids,  331 


Facial  motor  tract,  298 
Falx,  pig,  12.0  mm.,  275 

20. o  mm.,  327 
Fertilization  of  the  ovum,  38 
Fibrin,  in  chorion,  355 

in  decidua  reflexa,  349 
Filum  terminale,  75 
Fin  compared  with  limb,  257 
Floor-plate,  72 
Fluid,  Bouin's,  379 

Flemming's,  379 

Hermann's,  380 

Miiller's,  379 

Parker's,  379 

Tellyesnicky's,  378 

Zenker's,  378 
Foramen  epiploicum,  290 

of  Monro,  329 
defined,  74 


Fore-brain,  chick,  180,  201 

differentiation,  74 

origin,  72 

pig,  9.0  mm.,  253 

12  .  o  mm.,  271,  292 
Fore-gut,  chick,  180,  181,  187 

general  account,  60 

origin,  57 
Formalin,  378 
Fovea  cardiaca,  chick,  190 

origin,  57 
Furrow,  dorsal,  68 

Gall-bladder,  pig,  14.0  mm.,  288 
Ganglia,  auditory,  division  of,  79 
origin,  72 
pig,  9. o  mm.,  250 

12. o  mm.,  261,  283,  301 
17.0  mm.,  205 
20.  o  mm.,  214 

Ganglion,  acustico-facial,  293,  295,  298 
jugular,  293,  296 
nodosum,  277,  293 
petrosal,  293 

trigeminal,  262,  293,  295,  297 
Ganglionic  crest,  72 

chick,  180,  185,  188,  191 
Ganoids,  7 

Genetic  restriction,  14 
Germ-cells,  general  account,  25 

of  Acanthias,  26 

Germ-layers, -general  account,  18 
specific  quality,  19 
tissues  from,  19 
Germinal  area,  96 

wall,  65 

Gibbon,  ovum  in  third  stage,  131 
Gill-clefts,  62 

chick,  first,  201 
second,  202 
third,  205 
human,  140—147 
pig,  6.0  mm.,  248 

9.0  mm.,  257,  259 
12.0  mm.,  271,  273,  275,  277 
Gill-pouches,  origin,  62 
Glands,  classification,  23 
general  account,  21 
genital,  pig,  12.0  mm.,  287 
17.0  mm.,  308 
20. o  mm.,  322 
Globules,  polar,  37,  161,  162 
Gray  layer,  263,  270 
defined,  74 


INDEX. 


397 


Groove,  primitive,  48 
Growth,  law  of  unequal,  24 
of  embryo,  49 


Head-bend,  223 
Head-process,  53 

Heart,  chick,  186,  187,  188,  203,  205,  206 
general  account,  189 

origin,  96. 

pig,  12 .  o  mm.,  282 
Heidenhain's  hematoxylin,  382 
Hemispheres,  cerebral,  pig,  12.0  mm.,  275 
20.  o  mm.,  325,  329 

origin,  74 

Hensen's  knot,  48,  170 
Heredity,  theory  of,  28 
Hermaphrodites,  27 

pseudo-,  32 
Hind-brain,  chick,  180,  190,  191,  203 

differentiation,  74 

origin,  72 

pig,  9.0  mm.,  258 

12. o  mm.,  263,  292,  296 
Hind-gut,  general  account,  61 

origin,  57 

Hormone  theory  of  sex,  2  7 
Human  embryo,  118 

age,  calculation  of,  118 

classification  of  stages,  119 

Coste,  138 

Dandy,  58,  137 

Eternod,  136 

Frassi,  119 

His,  E,  136 

Lg,  141 

Sch,  141 

SR,  136 

2.15  mm.,  141 

2  .  6  mm.,  143 

3.2  mm.,  145 

4  .  o  mm.,  147 

4  .  2  mm.,  144 
Kollmann,  137 
Mall,  147 
Peters,  128 
Spec,  i.  54. mm.,  135 
stages,  classification  of,  119 

first,  119 

second,  119,  128 

third,  119 

fourth,  119,  134 

fifth,  120,  136 

sixth,  1 20,  137 


Human  embryo,  stages,  seventh,  121,  140 

eighth,  121,  141 

ninth,  121,  143 

tenth,  121,  146 

eleventh,  121,  147 

four  weeks  to  four  months,  148-159 

relations  to  uterus,  124 
Hyoid  cavity,  87 
Hypophysis,  pig,  12.0  mm.,  292 

24.0  mm.,  335 
vertebrates,  5 


Ichthyopsida,  7 
Impregnation  of  the  ovum,  38 
Infundibular  gland,  pig,  12.0  mm.,  268,  292 

24.0  mm.,  335 

Inner  man  in  segmentation,  44,  47 
Insectivora,  8 
Intermediate  cell-mass,  85 
Intervertebral  discs,  pig,  12.0  mm.,  301 

20. o  mm.,  314 
Intestine,  caudal,  213 

open  in  chick,  191,  210,  212 

pig,  9.0  mm.,  252,  255,  257 

17.0  mm.,  308,  311 
Iris,  331 

Iron  hematoxylin,  382 
Isthmus  of  brain,  origin,  74 

pig,  12.0  mm.,  292 
Iter,  defined,  74 


Jakobson's  organ,  326 

Kidney,  origin  of  renal  anlage,  309 

pig,  17.0  mm.,  308 

20. o  mm.,  322 

Knot,  Hensen's,  48,  170 

primitive,  48 
Kopffortsatz,  53 


Lachrymal  groove,  222,  224 
Lamina  terminalis,  335 
Larynx,  anlage  of,  276,  293 
Lateral  roots,  270,  297 
Layer,  subzonal,  44 
Lens  of  eye,  chick,  201 

origin,  78 

pig,  12.0  mm.,  271 
20. o  mm.,  329 
24.0  mm.,  333 
Lesser  peritoneal  space,  290,  301 


- 


398 

Leucocytes,  general  account,  96 
Limbs,  pig,  9.0  mm.,  257 
f  2  .o  mm.,  279 
20. o  mm.,  318 
vertebrates,  4 
Liver,  chick,  207 

general  account,  107 
pig,  9.  o  mm.,  252 
12 .  o  mm.,  288 
20.  o  mm.,  324 
24.0  mm.,  337 
vertebrates,  5 
Lungs,  pig,  9.0  mm.,  252 
12  .o  mm.,  285 
17.0  mm.,  306 
20. o  mm.,  317,  322 
Lutein,  35 
Lymph-glands,  105 
Lymph-sacs,  105 
Lymphatic  spaces,  pig,  20.0  mm.,  315 

system,  105 
Lyons  blue,  383 


Mblleus,  330 
Mallory's  stain,  385 
Mammary  anlage,  320 

bodies,  292 

Mandibular  cavity,  87 
Marsipobranchs,  7 
Marsupials,  8 
Mass  and  surface,  20 
Maxillary  process,  224 

pig,  20. o  mm.,  325 
Maxillo-turbinal  fold,  '311,  326 
Meatus,  external  auditory,  271 
Meckel's  cartilage,  329 

diverticulum,  57 
Mediastinum,  317 
Medulla  oblongata,  pig,  12.0  mm.,  296 

20. o  mm.,  336 
Medullary  canal,  differentiation,  7  r 

origin,  69 

stratification,  74 

structure,  69 
groove,  chick,  178,  180,   194 

human,  136 

origin,  68 
plate,  human,  134 

origin,  67 

Membrana  serosa,  64 
Menstruation,  339 
Mesectoderm,  185 
Mesencephalon,  chick,  180 


INDEX. 


Mesenchyma,  denned,  18 

histogenesis,  89 

pig,  12  .  o  mm.,  266 
17.0  mm.,  303 
Mesoderm,  chick,  218 

defined,  18 

early  history,  79 

origin,  51 

somatic,  defined,  81 

splanchnic,  192 
defined,  81 

Mesonephros,  defined,  109 
Mesothelium,  origin,  81 
Metamerism,  2 
Metanephros,  origin,  no 
Metencephalon,  origin,  74 
Microtomes,  389 

knives  for,  391 
Mid-brain,  chick,  r8o,  184 

differentiation,  74 

origin,  72 

pig,  9.0  mm.,  253 

12  .  o  mm.,  292 
Milk-line,  225,  226 
Monkey,  ovum  in  second  stage,  127 
Monotremes,  8 
Mouth  of  vertebrates,  5 
Mullerian  ducts,  pig,  20.0  mm.,  317 
Muscle-plate,  86 

pig,  9.0  mm.,  254 
Muscles,  hyoglossal,  329 

of  eye,  329 

origin,  89 

Myelencephalon,  origin,  74 
Myotomes,  pig,  12.0  rnm.,  273 


Nasal  pits,  76 

pig,  12  .  o  mm.,  277 
Naso-turbinal  fold,  326 
Neck-bend,  223,  292 
Necrobiosis,  15 
Nephrotome,  chick,  193,  212 

differentiation,  86 

origin,  85 
Nerve  fibers,  pig,  12.0  mm.,  263 

roots,  origin  of,  270,  280 
Nerves,  acoustic,  295,  298  ^r 

cervical,  273,  275,  278 

facial,  pig,  12.0  mm.,  271,  295,  298 

fourth,  pig,  12.0  mm.,  264,  268 

glosso-pharyngeal,  274,  295 

hypoglossal,  277,  296 

inferior  maxillary,  274 


INDEX. 


399 


Nerves,  olfactory,  76,  327 

optic,  77,  295 

origin,  72 

spinal,  280 

spinal  accessory,  268,  277,  296 

superior  maxillary,  326 

third,  pig,  12.0  mm.,  264,  268 

trigeminal,  295,  297 

vagus,  285,  29^6,  317 
Nervous  system,  origin,  67 

vertebrates,  4 
Neuraxons,  270 
Neuroblasts,  72,  73,  270 
Neuroglia  layer,  outer,  263 
Neuromeres,  pig,  6.0  mm.,  246 

12  .o  mm.,  264 
Neuropore,  anterior,  69,  178 
Nodulus  thymicus,  27<57~295 

origin,  63 
Notochord,  anlage  'of,  55 

growth,  55 

pig,  12  .o  mm.,  273         . 
17.0  mm.,  305 
20  .o  mm.,  315 
24 .o  mm.,  336 

relation  to  axial  mesoderm,  56 

ultimate  fate,  55 

vertebrates,  4 
Notochordal  canal,  53 
Nuclei  pulpqsi,  56,  337 

(Esophagus,  origin,  57 

pig,  12. o  mm.,  280,  285 

17.0  mm.,  305,  306 

20.0  mm.,  314,  317 

Olfactory  nerves,  origin,  76 

plate,  pig,  6.0  mm.,  24-9 

12  .  o  mm.,  277 

Omentum,  pig,  12.0  mm.,  288,  290 
Operculum,  pig,  259 
Optic  chiasma,  292,  335 

vesicles,  chick,  133,  180,  183,  202 

differentiation,  77 
Oral  plate,  58 

chick,  184 
human,  142 
Orange  G,  383 
Organsf*constitution  of,  20 
Otocyst,  chick,  199,  200 
general  account,  78 

pig,  12.0  mm.,  261,  263,  267,  293,  299 
Ova,  primitive,  26 
Ovulation,  35 
mouse,  161 


J 


Ovum,  before  maturation,  34 
constitution,  12 
fertilization,  mouse,  163 
gibbon,  third  stage,  131 
holoblastic,  10 
human,  34 

second  stage,   128 

Peters' s,  128 
impregnation,  38 
isotropism  of,  1 2 
maturation,  36 
meroblastic,  10 
monkey,  second  stage,  127 
mosaic  theory  of,  12 
segmentation,  42 

mammals,  160 

mouse,  160,  185 


Palate,  311 

cleft,  91,  312 
Panchoroid,  266,  321 
Pancreas,  general  account,   107 
pig,  12.0  mm.,  290 
24.0  mm.,  337 
Pangenesis,  29 
Parablast,  64 

Parathyroid  glands,  63,  315 
Penis,  pig,  20.0  mm.,  320 
Pericardial  cavity,  chick,  181,  209 

origin,  87 

Pericardial  epithelium,  pig,  12.0  mm.,  282 
Perichondrium,  origin  of,  304 
Peritoneal  membrane,  pig,  12.0  mm.,  286 

20. o  mm.,  322 

Peritoneum,  pig,  20.0  mm.,  322 
Pharynx,  chick,  184,  205 
general  account,  61 
origin,  57 

pig,  12.0  mm.,  237,  238,  269,  274 
vertebrates,  3 

Pia  mater,  pig,  9.0  mm.,  2^4 
12.0  mm.,  266 
17.0  mm.,  305 
20. o  mm.,  325 

Pig  embryo,  anatomy,  general,  228 
7.8  mm.  stage,  228 
12.0  mm.  stage,  231 
form,  external,  22 1 
7 .  5  mm.,  221 
10 . o  mm.,  223 
15.0  mm.,  225 
20. o  mm.,  226 
methods  of  obtaining,  219 


400 


INDEX. 


Pig  embryo,  sections,  diagram  of,  260 
selection  of  stages,  221 
serial  sections,  220 
studied  in  sections,  6.0  mm.,  246 
9.0  mm.,  250 
12.0  mm.,  259 
17.0  mm.i  303 
20. o  mm.,  311 
24 .  o  mm.,  330 
viscera  dissected,  231 
Pituitary  body,  268,  292,  336 
Placenta,  allantoic,  defined,  113 
at  seven  months,  345,  352 
chorionic,  defined,  113 
cotyledons  of,  362 
general  description,  359 
in  situ,  352 

intervillous  spaces,  363 
vessels  of,  360 
Placentalia,  8 
Plakodes,  chick,  217 

general  account,  76 
Plate,  closing  of. gill-cleft,  271 
Pleural  cavity,  322  * 

origin,  87 

Pleuro-peritoneal  space,  87 
Plexus,  brachial,  280,  ,283 
lateral  choroid,  328 
lumbar,  318 
Polar  globules,  37 
mouse,  first,  161 
second,  162 

Post-branchial  bodies,  63 
Premandibular  cavity,  87 
Primates,  8 
Primitive  axis,  52 

groove,  chick,  178,  197 
streak,  50 

Pro-amnion,  defined,  80 
Prochorion,  45 
Pronephros,  defined,  108 
Pronuclei,  fusion  of,  41 

mouse,  164 
Pronucleus,  female,  37 

mouse,  163 
male,  39 

mouse,  163 

Prosencephalon,  chick,  180. 
Proto vertebrae,  defined,  84 
Pupil  of  eye,  331 


Rabbit  embryo  with  eight  segments,  179 
Recapitulation,  law  of,  29 


Reconstructions,  by  drawings,  385 

by  wax  plates,  387 
Reduction  division,  37 
Regression,  15 

Restriction,  law  of  genetic,  14 
Rhombencephalon,  chick,  180 
Rodents,  8 


Sauropsida,  7 

Sclerotome,  pig,  6.0  mm.,  250 

Sections,  orienting,  388 

paraffin,  mounting  of,  381 

staining,  381 
Segmental  vesicle,  86 

zone,  178 
Segmentation  nucleus,  41 

of  the  ovum,  42 
in  Limax,  43 

spindle,  42 

Segmented  animals,  2 
Segments,  chick,  third,  192 

general  morphology,  2 

occipital,  1 80 

primitive,  84 
Sella  turcica,  336 
Sense-organs,  5 
Septum,  nasal,  312,  326 

transversum,  chick,  190,  209 

relation  to  ccelom,  87 
Sex,  27 

cause  of,  28 

cells,  26 
Sexual  characteristics,  secondary,  27 

cords,  322 

Shield,  embryonic,  44,  47 
Sinus,  cavernous,  268 

cervicalis,  222,  223 
human,  147—149 
pig,  6.  o  mm.,  249 
12  .  o  mm.,  275 

lateral,  265 

rhomboidal,  69 

chick,  178,  1 80 

superior  longitudinal,  265,  275 

terminalis,  of  chick,  91,  97 

venosus,  origin,  98 
Sinusoids,  of  heart,  283 

of  liver,  252,  289,  324 

of  suprarenals,  338 

of    Wolffian  body,  256,  287,  306,  307 

origin  in  liver,  209 
Skull,  anlage  of,  325 
Somatic  cavity.     See  Splanchnocele. 


INDEX. 


401 


Somatopleure,  7,  82 
chick,.  187,  211 
pig,  12. o  mm.,  282,  286 

20.0  mm.,  318 
Somites,  chick,  192,  207,  212 
defined,  84 
differentiation,  85 
general  morphology,  2 
origin,  84 
secondary,  86. 

pig,  6.0  mm.,  246,  250 
Spermatozoon,  33 

entrance  into  ovum,  38 

mouse,  163 
Spinal  cord,  73 

chick,  205 
differentiation,  75 
pig,  6.  o  mm..  246 
12  .o  mm.,  269 
17.0  mm.,  305 
20 .  o  mm.,  312 
Splanchnocele,  84,  87 

pig,  12.0  mm.,  287 
Splanchnopleure,  7,  82 
chick,  187,  203,  211 
pig,  12.0  mm.,  288 
Spongioblasts,  73 
Staining,  methods  of,  381 
Stigma  of  Graafian  follicle,  35 
Stomach,  origin,  57 

pig,  12  .o  mm.,  288 
Stomodaetini,  184 
Streak,  primitive,  48 

rabbit,  170—173 
Striae  acusticae,  268 
Subzonal  layer,  44 

Suprarenal  capsule,  pig,  24.0  mm.,  337 
Surface  and  mass,  20 
Sympathetic  system,  cervical,  279,  283 
pig,  17.0  mm.,  305,  308 
20 .  o  mm.,  314 


Telencephalon,  origin,  74 

Teleosts,  8 

Testis,  322 

Tetrads,  36 

Thyroid  gland,  origin,  63 

pig,  12.0  mm.,  277 
20. o  mm.,  315 
Tissue,  classification  of,  19 
Tongue,  pig,  12.0  mm.,  293 
20. o  mm.,  311,  327 
Tonsil,  origin,  63 
26 


Trachea,  pig,  12.0  mm.,  2 76,  2 79,  280 

20. o  mm.,  314,  317 
Trigeminal  tract,  263,  297 
Trophoblast,  (footnote)  44 
Trophoderm,  degeneration,  366 

early  stage  described,  364 

general  account,  114 

origin,  47 
Tubal  band,  318 
Tuber  cinereum,  292 
Tunica  albuginea,  322 

vasculosa  lentis,  331,  333 
Tunicata,  9 


Umbilical  cord,  amnion  from,  115 
general  account,  115 
human,  at  seven  months,  345 
ectoderm  of,  375 
mesoderm  of,  374 
study  of,  372 

opening,  pig,  9.0  mm.,  253 
Umbilicus,  pig,  9.0  mm.,  254 
Unguiculates,  8 
Ungulates,  8 
Urodela,  8 
Urogenital  ducts,  5 

general  account,  no 
ridge,  of  vertebrates,  5 
Uterus,  general  histology,  339 

human,  pregnant,  two  stages  of,  124 
menstrual  changes,  339 
pregnant,  two  stages  of,  341 
at  three  months,  343 
at  seven  months,  345 
Uvea,  332 


Valves,  Eustachian,  282,  301 
sinistral,  301 
Thebesian,  282 
Veins,  anterior  cardinal,  origin,  102 

pig,  12.0  mm.,  268 
cardinal,  chick,  200,  207,  210 

pig,  12.0  mm.,  264,  268,  28; 
17.0  mm.,  306 
20. o  mm.,  317 
common,  origin  of,  98,  103 

pig,  12  .o  mm.,  282 
primitive  arrangement,  98 
iliac,  320 

inferior  cava,  origin,  104,  257 
jugular,  279 
maxillary,  293 


402  INDEX. 

Veins,  jugular,  pig,  12.0  mm.,  268  Vesicles,  ammo-cardiac,  87 

20.  o  mm.,  314  chick,  180,  186 

lateral  cardinal,  265  cerebral,  origin,  71 

of  the  head,  268  optic,  origin,  71 

lingual,  279  segmental,  86 
omphalo-mesaraic,    chick,     178,     180,    190,       Villi,  branching,    367 

191,  208,  210  histology  of,  356 

origin,  93  of  allantois,  253,  324 

primitive  arrangement  of,  98,  103  of  chorion,  367 

ophthalmic,  271  shape  of,  367 

peripheral  (of  limb),  320  vessels  of,    370 
portal,  pig,  9.0  mm.,  252 

12.0  mm.,  290  Weigert's  hematoxylin,  384 

posterior  cardinal,  origin,  103  Weismannism,  29 

pulmonary,  285  Wolffian  body,  general  account,  109 

subcardinal,  256  pjg>  g  0  mm  _  252)  256 

superior  longitudinal  sinus,  265  120  mm     287 

mesenteric,  311  x  7   0  mm  |  3o6 

umbilical,  origin,  104  2O   o  mm  _  322 

pig,  9.0  mm.,  257  duct,  chick,  212 

12.0  mm.,  289  tubules,  origin,  86 
17.0  mm.,  311 
vitelline,  pig,  9.0  mm.,  253 

1 7  .  o  mm.,  3 1 1  Yolk-  Absorption  of,  65 

Velum  transversum,  origin,  74  D     'cavity>  53 

Vena  capitis  lateralis,  268,  299  Yolk-sac,  angioblast  of,  66 

cava  inferior,  300  entoderm  of,  64 

development,  257  formed  by  splanchnopleure,  82 

hepatica  communis,  pig,  9.0  mm.,  252  Seneral  morphology,  63 

Venous  system,  102  human'  66 

Ventral  roots,  270,  283  structure,  375 

Ventricle,  fourth,  pig,  6.0  mm.,  246  m  umblhcal  cord-  374 

12.0  mm.,  293  mesoderm  of,  66 
lateral,  pig,  12.0  mm.,  275 

20. o  mm.,  327                      .  Zimmermann's  arch,  101 

of  heart,  pig,  12.0  mm.,  282  Zona  pellucida,  34 

Vertebrae,  pig,  12.0  mm.,    303  radiata,  34 

17.0  mm.,  304  Zones,  dorsal,  72 

20. o  mm.,  314,  315  of  His,  72 

24.0  mm.,  337  parietal,  defined,  84 

Vertebrate  type,  2                                                              .  segmental,  178,  193,  213 

fundamental  characteristics  of,  3  defined,  84 

modifications  of,  7  ventral,  72 


•    4 


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QCT     81947 


>  1947 


0  1991 


219SS 

SEP  2  9  1955 


MQV  2 1 1362 


Mo?  9  "=;;•••••  - 


-rtrrc 





LD  21-5rn-l,'39(7053s7) 


U.C.  BERKELEY  LIBRARIES 


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